12-18 year old can have sex with people no more than 1 and a half year older/younger than them. After that who cares (tbh i broke this rule by fucking a 40 year old milf last summer).
18 or 21. And it should be for everything not just certain things. So either raise it to 21 along with joining the military (they won't) or lower drinking and smoking back to 18. Which probably won't happen, but is still more likely than them limiting military recruitment.