There are pros and cons for either demographic: whilst being older may be ideal since not only you bypass age restrictions, you also have the capability on purchasing it with your own money without having to ask parents for that given that you have employment and recurring income. Gaming as a kid gives more time without having to dwell much on responsibilities that adults have to worry about but at the same time are held by school.
The pros of gaming as an adult is that you now have the money to go buy all the games you wanted to play as a kid. The cons are that you no longer have the time.
I have to say that the lack of responsibility and abundance of time makes gaming as a kid better.