Maturity is a value judgment. At best. At worst it's simply a power move. There's no objective way to measure your brain juices and say now you're "fully developed" or whatever.

People eager to define other people as insufficiently adult adults, should be viewed with the same skepticism as people who want to put their political opponents in an asylum.

If you think it's a problem that young adults today play too much video games or whatever, take the ball and not the man. The problem then is in the behavior, not in people's essence. The youth are as bad as every generation complains that they are, no more, no less.