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.
It's ridiculous that some kids get their post secondary education degree without ever having held a job (of any kind).
I find it amazing that teens walk around with $200-$2000 worth of electronics that their parents paid for them with no thought to their value because they didn't pay for it, and they know they'll be replaced if they whine enough.
The case for upping the age of maturity to 25 is that there is solid brain science suggesting that that is how old we are when our brain finishes developing the ability to make mature decisions. See http://www.hhs.gov/opa/familylife/tech_assistance/etraining/... for that.
However you're absolutely right that children who are never given responsibility never develop their ability to take responsibility.
I wouldn't say that increasing it to the age of 40 would be crossing the maturity threshold, either.
The main reason teen rebellion and delinquency are issues in the first place is because young people are already held back for too long.
An arbitrary age of maturity has nothing to do with actual (as opposed to legal) adulthood or responsibility. The fact is that "kids" at the age of 17 and 18 are already making decisions that will affect them for the rest of their lives. Making the legal age later will certainly not help these people realize that the decisions they are making are important or that they need to take responsibility for themselves.