I'm 20 right now, and I'm at the point in my life where maybe I should just 'grow up' as well and stop MMOs.
I'm 33, played FF11 from 25-30.
You're never too old to do what's fun for you. You just have to have balance with it.
Having fun and being addicted are 2 entirely different things.
I have an addictive personality myself, but from time to time I have the ability to "Cold turkey" whatever it is I'm obsessing over. I quit 11 after spending over 40 hours a week playing and realizing I had just sunk 40 hours into something I wasn't getting paid for, lol.
After a few months off game I picked up City of Heroes, played for 2 years, dropped it, after a few more months I picked up WoW, I'm on it now(sadly it's all that was left I exhausted all my free trial options.
I like to think I balance my addictions and life pretty well now. I no longer play 40 hours in a week, unless I'm off work for some reason, hehe.
I have a job, a Fiance who is understanding(She has a couple non gaming obsessions, so we let each other have our vices), and a blossoming career as an Artist(Not pro yet, but getting paid gigs)
I do most of my gaming during winter. I do Sidewalk chalk art so during the warmer months I'm out on the town annoying officials and coloring the city. When it's too cold for sidewalks I fire up my gaming addiction and get hardcore for a few months, then drop off again when it warms.
I know I'm addicted but I kind of revel in it because i know I have it balanced, so really I'm looking forward to XIV and the feeling of that first sweet hit when I enter my starting city and find my way out into the wide open imaginary world.
Growing up just means balancing the things you want to do with the things you NEED to do.
As long as you're meeting your responsibilities and do whatever you are obligated to do regularly, whether it be bills or school or family, you don't have to give anything up.
My old tabletop friends and I had a solid rule to make sure everyone kept perspective. The rule was, if we had a game, no matter when the game was scheduled, it was secondary to chicks. If a player had the option to go out on a date during the time of a scheduled game, they were not only excused but if they chose to come to the game instead of going out with a real live girl, they were out of the game.
Some adults paint their faces and act like 5 year olds when they go to football games, no one tells them to grow up. Other adults scream like little girls when their favorite American Idol moves on to the next round. These people, as immature as they can be sometimes, are still adults, and so are we. We are adults who play games, and as long as nothing gets left by the wayside to fulfill your love of gaming then you're fine.