Here is a link. The primary information I have from Dr. Phil, somebody I trust in giving correct information and stating where his information comes from. But this is another source where it is explained:
Society for Neuroscience | The Adolescent Brain Other parts of the brain also undergo refinement during the teen years. Areas associated with more basic functions, including the motor and sensory areas, mature early. Areas involved in planning and decision-making, including the prefrontal cortex -- the cognitive or reasoning area of the brain important for controlling impulses and emotions -- appear not to have yet reached adult dimension during the early twenties. The brain's reward center, the ventral striatum, also is more active during adolescence than in adulthood, and the adolescent brain still is strengthening connections between its reasoning- and emotion-related regions.
As adults we are responsable for protecting children. This also means stopping them from making decisions that can affect their entire life and they cannot oversee the consequenses yet.
Sometimes that might be wrong, the adult may make the wrong decision, but I think that the child can change that when they reach adulthood. If the child (teenager) made a decision when they were young, it is usually to late to change anything about it.
Lets take for example having a baby (lets leave the sex out of it for arguements sake).
A child of 15 might really really want to have a baby. It is very very important for her. She wants something to love, and she feels it is so important that it cannot wait.
her parents disagree, they can see a bigger picture and more consequenses down the line. The forbid her to have a child.
Now, if the child doesnīt listen and has a child she is stuck with it for at least 20 years. Which is ok if she doesnīt change her mind, but not so much if she does change her mind.
In case the child listens to her parents, and doesnīt have a child, maybe at age 16 she wants a dog... she gets one, is very happy, meets the love of her life at 20 and they have a child together...
If the parents are wrong and she is serious of having a child, nothing stops her from having one at age 18... at least it will give her time to really thing about the responsabilties.
It is a tactic that my parents used for us. If we really wanted something we had to wait at least a week (or a month for something big) and if we still wanted it, we could have it. 9 times out of 10, we completely forgot about it...
But.. I still donīt know where old age stops you from doing something you want to do...? Or are you just talking socially?