Age Limits?

My mom always tells me that I won't be an adult until she has taken me off of her insurance when I turn 24. Well, 26 now, thanks to the brilliant new Obama health care bill. But the age of legal "independence" is 18. I find it interesting that we have always had legal age minimums but no age maximums. You have to be at least 16 to drive, 18 to vote, 21 to drink, etc. But there is no age at which you must stop driving, voting, drinking. If I'm too young and inexperienced to vote at 17, is it possible that someone at say 77 would be too old and not in their right mind to do the same. I'm no ageist, but people mature at different ages. One could be mature enough to vote, drink, drive, etc. before their legal age. Of course this would be the anomaly but they exist. Also, someone past a certain age cannot be expected to be able to drive with the same attention, or be mentally sound enough to make political decisions. And that's in the case they are sober. Add alcohol to the picture, and that would be an entirely different story.