The US is weird in that we can't really decide if you are an adult at 16 (can work a standard job, can drive), 18 (can vote, can serve in the military) or 21 (can drink). In fact for a while between the Vietnam War and the late 1980s, the drinking/smoking age was 18 in many states (or odd compromises where you could drink beer at 18 but liquor at 21). It was largely pressure from groups like MADD (Mothers Against Drunk Driving) that increased the drinking age.
Considering brain development and differing effects of various substances/activities, I don't see any contradiction with having different age thresholds.
Well being able to serve in the military at 18 around weapons that potentially kill but you can't have a bear for another 3 years does sound a bit contradictory.
