The main problem with questions like this one is their, on the one hand, ambiguity and, on the other hand, relentlessness with which those who ask them require to be answered. What kind of decisions? About what? If we think along the lines presupposed by the question, we should define the unified age when teenagers should be let to both choose what they would have for breakfast and whom to marry.
The point of view behind this question kind of presupposes that until certain age children and teenagers are like small kids who shouldn’t be let to do anything on their own, and afterwards immediately become responsible members of the society one may and has to trust.
What I think is that this approach to the problem is incorrect in the very least. People are different, children are different, circumstances are different. One person needs to be overseen all the time lest he does something stupid, another may be his own master at the age of ten and manage to do it really well.
In my opinion, both extremes are equally bad. When a teenager is left completely on his own, it may result not only in some personal problems – he may bring problems to his parents as well. But when he is under constant surveillance, this interferes him being an individual, as well as breeds rebellion and hatred towards his parents, which is hardly what they want to achieve.
Reasonable approach is to teach children to make right decisions. Give them maximum freedom in everything that cannot result in irrevocable mistakes. Teach them to be reasonable and responsible themselves, not only relying on you and your advice. Let them learn to be independent.