Or offer a middle ground - i.e.
15A - 12-14 with parental accompaniment, 15+ fine on their own
18A - 15-17 with parental accompaniment, 18+ fine on their own
(i.e. an "A" would be too strong to get the next certificate down, but with aqueduct parental supervision will be okay for many in the middle ground between certificates).
That sounds good, theres an 18A in Canada isnt there?
But unfortunately what is too strong for a 12 (Dark Knight etc) is subjective.