diff --git a/app/helpers/event_helper.rb b/app/helpers/event_helper.rb
index 25ac9f890a..92f4e8fdea 100644
--- a/app/helpers/event_helper.rb
+++ b/app/helpers/event_helper.rb
@@ -143,7 +143,7 @@ def resolve_answer_text(field, submitted_answer)
     case field&.field_identifier
     when *FormField::SECTOR_FIELD_IDENTIFIERS
       resolve.call(Sector)
-    when "primary_age_group", "additional_age_group"
+    when *FormField::AGE_GROUP_FIELD_IDENTIFIERS
       resolve.call(Category)
     else
       submitted_answer
diff --git a/app/models/form_field.rb b/app/models/form_field.rb
index c8ee2feba6..00dfa8b5c3 100644
--- a/app/models/form_field.rb
+++ b/app/models/form_field.rb
@@ -32,17 +32,28 @@ class FormField < ApplicationRecord
   # submitted value for these is a Sector id (as a string).
   SECTOR_FIELD_IDENTIFIERS = (ADDITIONAL_SECTOR_FIELD_IDENTIFIERS + PRIMARY_SECTOR_FIELD_IDENTIFIERS).freeze
 
+  # Single-select "primary age group" field identifiers. Primary has never been
+  # renamed, so this is just the canonical name — kept as a list for symmetry
+  # with the sector fields and the multi-select additional field below.
+  PRIMARY_AGE_GROUP_FIELD_IDENTIFIERS = %w[primary_age_group].freeze
+
+  # Multi-select "additional age group(s)" field identifiers. "additional_age_group"
+  # is the canonical name new forms are built with; the pluralized
+  # "additional_age_groups" is also accepted so older forms carrying it keep
+  # resolving (mirrors the legacy coverage the sector fields have).
+  ADDITIONAL_AGE_GROUP_FIELD_IDENTIFIERS = %w[additional_age_group additional_age_groups].freeze
+
+  # Every age-group field identifier, primary and additional.
+  AGE_GROUP_FIELD_IDENTIFIERS = (PRIMARY_AGE_GROUP_FIELD_IDENTIFIERS + ADDITIONAL_AGE_GROUP_FIELD_IDENTIFIERS).freeze
+
   # Field identifiers whose selectable options are sourced dynamically from a
   # CategoryType's published categories. The submitted value is a Category id
   # (as a string). Maps the field identifier to its backing CategoryType name.
-  # Both the "primary" and "additional" age group fields are backed by the
-  # published AgeRange categories. Unlike the sector fields, age groups have no
-  # catch-all option — the additional sector field keeps "Other", but neither age
-  # field offers one.
-  DYNAMIC_FIELD_CATEGORY_TYPES = {
-    "primary_age_group" => "AgeRange",
-    "additional_age_group" => "AgeRange"
-  }.freeze
+  # Every "primary" and "additional" age group field (canonical or legacy) is
+  # backed by the published AgeRange categories. Unlike the sector fields, age
+  # groups have no catch-all option — the additional sector field keeps "Other",
+  # but no age field offers one.
+  DYNAMIC_FIELD_CATEGORY_TYPES = AGE_GROUP_FIELD_IDENTIFIERS.index_with { "AgeRange" }.freeze
 
   # The payment-method field. Its answer options ("Credit card (now)", etc.) are
   # wired to Stripe charge logic in the controllers, so they must not be edited
diff --git a/app/models/person.rb b/app/models/person.rb
index 424dd07d15..10b249a761 100644
--- a/app/models/person.rb
+++ b/app/models/person.rb
@@ -230,7 +230,7 @@ def remote_search_label
 
   # Field identifiers whose "Other" free text maps onto the category-backed
   # profile fields shown on the edit page.
-  OTHER_WORKSHOP_SETTING_IDENTIFIERS = %w[primary_age_group additional_age_group].freeze
+  OTHER_WORKSHOP_SETTING_IDENTIFIERS = FormField::AGE_GROUP_FIELD_IDENTIFIERS
 
   # Free-text "Other" sectors the person typed on registration forms.
   # They can't be Sector records, so they're surfaced beside the sector tags.
diff --git a/app/services/event_registration_services/public_registration.rb b/app/services/event_registration_services/public_registration.rb
index ff7c3e3e82..021832b852 100644
--- a/app/services/event_registration_services/public_registration.rb
+++ b/app/services/event_registration_services/public_registration.rb
@@ -273,10 +273,10 @@ def create_agency_address(organization)
     end
 
     def assign_tags(person, organization)
-      primary_sector_ids = collect_sector_ids(FormField::PRIMARY_SECTOR_FIELD_IDENTIFIERS)
-      additional_sector_ids = collect_sector_ids(FormField::ADDITIONAL_SECTOR_FIELD_IDENTIFIERS)
-      primary_age_ids = collect_ids_from_checkboxes("primary_age_group")
-      additional_age_ids = collect_ids_from_checkboxes("additional_age_group")
+      primary_sector_ids = collect_ids(FormField::PRIMARY_SECTOR_FIELD_IDENTIFIERS)
+      additional_sector_ids = collect_ids(FormField::ADDITIONAL_SECTOR_FIELD_IDENTIFIERS)
+      primary_age_ids = collect_ids(FormField::PRIMARY_AGE_GROUP_FIELD_IDENTIFIERS)
+      additional_age_ids = collect_ids(FormField::ADDITIONAL_AGE_GROUP_FIELD_IDENTIFIERS)
 
       if primary_sector_ids.any? || additional_sector_ids.any?
         person.tag_sectors(primary_ids: primary_sector_ids, additional_ids: additional_sector_ids)
@@ -292,7 +292,7 @@ def assign_tags(person, organization)
       end
     end
 
-    def collect_sector_ids(identifiers)
+    def collect_ids(identifiers)
       identifiers.flat_map { |id| collect_ids_from_checkboxes(id) }
     end
 
diff --git a/spec/requests/events/professional_field_identifiers_spec.rb b/spec/requests/events/professional_field_identifiers_spec.rb
index d9581741f7..7848e2a969 100644
--- a/spec/requests/events/professional_field_identifiers_spec.rb
+++ b/spec/requests/events/professional_field_identifiers_spec.rb
@@ -29,19 +29,23 @@
   let!(:sector_other)     { create(:sector, :published, name: Sector::OTHER_SECTOR_NAME) }
   let!(:sector_hidden)    { create(:sector, name: "Hidden sector") }
 
-  # Each scheme maps the two sector fields onto a canonical or legacy identifier;
-  # the age-group fields have never been renamed, so they stay constant.
+  # Each scheme maps the two sector fields and the additional-age field onto a
+  # canonical or legacy identifier. The primary age field has never been renamed,
+  # so it stays constant; the additional age field also accepts a pluralized
+  # legacy name (`additional_age_groups`) some older forms carry.
   {
-    "canonical identifiers"         => { primary_sector: "primary_sector_single",       additional_sector: "additional_sectors" },
-    "legacy additional-sector name" => { primary_sector: "primary_sector_single",       additional_sector: "primary_sector" },
-    "legacy service-area names"     => { primary_sector: "primary_service_area_single", additional_sector: "primary_service_area" }
+    "canonical identifiers"          => { primary_sector: "primary_sector_single",       additional_sector: "additional_sectors" },
+    "legacy additional-sector name"  => { primary_sector: "primary_sector_single",       additional_sector: "primary_sector" },
+    "legacy service-area names"      => { primary_sector: "primary_service_area_single", additional_sector: "primary_service_area" },
+    "pluralized additional-age name" => { primary_sector: "primary_sector_single",       additional_sector: "additional_sectors", additional_age: "additional_age_groups" }
   }.each do |scheme_name, ids|
     context "with #{scheme_name}" do
+      let(:additional_age_identifier) { ids.fetch(:additional_age, "additional_age_group") }
       let(:form) { build_professional_form(ids) }
       let(:primary_sector_field)    { form.form_fields.find_by!(field_identifier: ids[:primary_sector]) }
       let(:additional_sector_field) { form.form_fields.find_by!(field_identifier: ids[:additional_sector]) }
       let(:primary_age_field)       { form.form_fields.find_by!(field_identifier: "primary_age_group") }
-      let(:additional_age_field)    { form.form_fields.find_by!(field_identifier: "additional_age_group") }
+      let(:additional_age_field)    { form.form_fields.find_by!(field_identifier: additional_age_identifier) }
 
       before { EventForm.create!(event: event, form: form, role: "registration") }
 
@@ -98,7 +102,7 @@
             ids[:primary_sector] => sector_education.id.to_s,
             ids[:additional_sector] => "#{sector_mh.id}, Other: Equine therapy",
             "primary_age_group" => age_adults.id.to_s,
-            "additional_age_group" => "#{age_teens.id}, #{age_children.id}"
+            additional_age_identifier => "#{age_teens.id}, #{age_children.id}"
           )
         end
 
@@ -150,8 +154,9 @@
   # ---- Form construction ----
 
   # Builds a registration form with just the identity + professional sections,
-  # then renames the two sector fields onto the scheme's identifiers (the age
-  # fields keep their canonical names — they were never renamed).
+  # then renames the two sector fields (and, when the scheme asks, the additional
+  # age field) onto the scheme's identifiers. The primary age field keeps its
+  # canonical name — it was never renamed.
   def build_professional_form(ids)
     form = FormBuilderService.new(
       name: "Reg #{ids[:primary_sector]} / #{ids[:additional_sector]}",
@@ -160,6 +165,7 @@ def build_professional_form(ids)
     ).call
     rename_field(form, "primary_sector_single", ids[:primary_sector])
     rename_field(form, "additional_sectors", ids[:additional_sector])
+    rename_field(form, "additional_age_group", ids.fetch(:additional_age, "additional_age_group"))
     form
   end