I stand corrected... So, although the age limit varies by state and is not the same as the age of majority (gaining full legal rights), every state has an age limit below which sex is illegal in all circumstances; usually around 14, but 17 or 18 in a few states. The separately stated age above which all intercourse is valid (i.e., with much older people) is 16 in most states, never lower.

Wikipedia also has an interesting map of worldwide age of consent by country (not representing rules for age differentials or categories). Oh, and quite a few Western countries have laws against specific sexual acts or against certain numbers or sexes of participants.