Actually, that’s exactly what age of consent is. The age at which you’re allowed to do things with an adult of any age.

Romeo and Juliet (or close in age) exceptions are for the situation you’re describing, and are usually tacked onto age of consent laws as an exception.

In Canada, there’s a pair of these. At 14 and 15 it’s less than 5 years older, and at 12 and 13 it’s less than 2 years older.