How will that help? You don't solve the problem of people being immature even into adulthood by delaying the official onset of adulthood yet more. You solve it by treating teenagers as adults in training. Which used to be the case until very recently. Now we avoid giving responsibility to teens, we treat high school like day care or a prison, and we wonder why our "kids" don't magically acquire maturity when they turn 18, or 21, or 25, or 30.

Maturity and responsibility are skills that must be taught. And they cannot be taught at a distance, they must be taught live, with the real world, because it is only through learning that actions have consequences, sometimes serious ones, that maturity is acquired.