I recently went out with my girlfriend, who within the next year I will be engaged to, and somehow the topic game up that she thinks that I am too wrapped up in all my "games", as she puts them, and that she thinks that eventually I need to grow up and leave that stuff behind.
Not really knowing either of you, it's hard to make specific commentary on your situation in specific, but in general: if the person you intend to spend your life with goes beyond being indifferent to your personal hobbies to disapproving of them or even going so far as to be contemptuous of them, it will create an eventual problem. Playing games has nothing to do with maturity or the lack thereof.
I speak as a 42-year old who has been happily married to a female gamer for 17 years. We have two kids, now aged 13 and 10. Gaming is a big part of our lives and we use games as a way to spend time and bond together. One member of my gaming group, has a wife who no longer games, but she understands the hobby. One member of my group had a husband who had no interest in gaming at all, but considered it that player's hobby, just like his hobby was collecting 50's radio music.
This doesn't mean that I'm saying your relationship cannot work or that she's being particularly close-minded. I don't know her or you, so I have no idea. But my concern would be more that she has an expectation that you will surrender a part of yourself or give up something you cherish for...what, exactly? Gaming is no more incompatible with a busy lifestyle of work and family than being a super-fan of a local team, joining a quilt club, being an avid reader, painting, sky-diving, bicycling, going out drinking or any number of other hobbies.
In point of fact, at 21, I can't imagine why she thinks you need to 'hunker down' at all. From my perspective, you've got a few more years of 'not getting serious' about anything left in you.
A successful relationship is built on both trust and compromise. Good couples usually have both mutual interests and tastes as well as things that they don't share. My wife likes a lot pickled and spicy foods much more than I do. She likes horror movies more than I do, while I like crime procedurals that she can find tedious...we both like Kung-Fu movies. I mean, come on. My point is that I appreciate our differences as does she. We each get the other to try and do things we might not do on our own and sometimes discover that we like them. And when we don't we respect that and move on.
I introduced my wife to RPGs, but she'd already role-played...at midnight showings of Rocky Horror. I'd picked out nice clothes a few times, but my wife showed me how to match colors and dress better. I have my miniatures to paint and she has her quilting to do. We make each other better. After 17 years together, I'm still just as happy to come home to her as I was when we married...the only difference is that now I have two more people waiting for me.
Perhaps you should try and discuss with her why she thinks your gaming is something that needs to be grown out of. Is it the expense? The time you spend? Perhaps what she's REALLY saying is that she's feeling you spend an inordinate amount of time gaming and sometimes neglect attention to her. It could be that she feels your hobby is taking away from your time as a couple and it bothers her. And since I don't know you personally, she could be right. Either way, an honest dialogue about it would probably be helpful. This is not a conversation you want to have AFTER you get married. If you're worried about what she'll say, then you should know NOW.
Either way, I hope you can find a solution.