First instinct with this is to say, "What for?"
Because there are some things you can be adult enough for at sixteen, some at eighteen, and some at twenty one. Other things you might not be adult enough for until you're like...Sixty. And then you'd still be young.
if we take it as a statement of when you reach your majority...I'd say...Go for twenty one to twenty five. Most people when they are eighteen are still total morons or at least rather clueless as to important things such as money and dealing with the big bad world. When they reach twenty...three we'll say, they have mostly got a life plan, have been through either all or most of their higher/further education and have most of the braincells aligned in the right places. But I don't completely think that we should do like they do now and dump it all on you, when you reach a certain age and leave you to rot in hell or float in onto heaven.
I think they should give you....
....the right to vote at seventeen.
....the right to drive at eighteen.
.....the right to drink alcohol at eighteen.
.....and the right to be able to own/rent property of a house value at eighteen but the property should be in a controlled way i.e. a student flat or bedsit, not a twenty bedroom manor house.
.....the right to be considered an adult at everything by twenty
.....and the right to join the army at twenty one (I do not approve of teenagers signing up to be blown apart)
But that's just my view. Sorry if it offends you but it is. I think that people either want too much from teenagers at too a young age, or too little. Teenagers essentially live with no responsibilties of a great nature until they're eighteen and then they have a whole lot of them. I think the big people should be easing people into the role of adulthood and by twenty five, the role is completely cemented.
On that note, I thoroughly approve of Finland's idea of driving tests - you have to practise for about three years, and be able to do everything from driving at night, to proving your skills at dealing with harsh road conditions and problems like aqua-planing, and other road users before you're allowed to pass. Far safer than our ideas which are basically "Get in the car. Start it. Drive. Stop. Pass." After all there are too many accidents on the roads which involve young people and their cars because they've only just passed.