I am not sure how many of these apply to you, But, I am in my 50s and since I have been in the workforce, every year, I had to work with someone young, very young, let it be an intern or a fresh grad or an early starter likewise yourself. And these are my observations. Again nothing implying you do these, or any assumptions. Just observations. Feel free to pick and choose
Leave the jeans, raggedy shirts and flip-flops at home for the weekend. Dress office appropriate. A slack a dress shirt or a nice polo shirt with some dress shoes goes a long way.
Keep lord of the rings (or enter any young adult movie name here) memorablia for your room at home, instead of scattering them in your cubicle
Captain crunch might have been fine for breakfast last year, but if you have to eat at work, try having toast and butter or yogurt with fruit salad. Same for lunch. PB&J was when you were in grade school. Despite how much you like those things, do not eat them at the presence of your office co-workers.
Pay attention to what people are talking about in the coffee room or water cooler and even if you are genuinely disinterested, try to learn some talking points about them. Probably NFL football in season, college hoops during spring and baseball in the summer months. Do not butt in the conversations but when there is a silence while others talking, you can interject your own points. It helps to make you one of the team rather than an outsider.
Stay away from online chats or phone calls with your buddies from the age of empires clan, during the day time. reserve your home time for those chats. Gamer is still considered as child, although it is evolving.
If you are able to afford, move out of mom's basement or garage into a tiny one bedroom apartment. Occasionally, you can invite few people from work, for whatever reason, like catching the game or BBQ on the patio.
Biking to work is healthy and admirable but having a car with four wheels and a running engine is another admission to the big league. Again if you can afford it, it is a great point.
Try to mirror what your co-workers are doing to a reasonable extent, in order to be accepted as one of them not as just a kid.
Most importantly, remember, in a couple of years they will hire someone younger than you are and you will no longer be the "kid" The dues you are paying right now will go away for good. Hang in there.