added first version of a new function to check adult status at a certain date.

Author: CloevVeller

app/controllers/members/participants_controller.rb

@@ -176,7 +176,7 @@ def participant_limit_check?
   end
 
   def participant_alcohol_check?
-    @activity.is_alcoholic? && @member.underage?
+    @activity.is_alcoholic? && @member.underage_at?(@activity.start)
   end
 
   # [PATCH] /activities/:id/participants

app/models/member.rb

@@ -246,6 +246,11 @@ def underage?
     !adult?
   end
 
+  def underage_at?(event_date)
+    return birth_date.blank? ||
+       ((18.years.ago + (event_date - Date.today)) < birth_date)
+  end
+
   def master?
     educations.any? do |education|
       education.status == 'active' && Study.find(education.study_id).masters