Wow !!!
I never thought of "age" when I was your age. I thought about getting paid, laid and stoned...pretty much in that order as well.
I did not start living a healthy life style...eating right, quit smoking, no drugs, etc. until I was in my mid 20's. Went a bit nuts about it. Lost lots of weight, ate only very healthy foods, started running, became a competitive runner in 10k's and marathons, switched over to bicycling and raced for the next 18 years until a bad crash ended my career.
It is not over, only the beginning but it is up to you to do "something".
Find an activity and do it.
Don't think about your age.
I am 51 now and do not feel or look it....however I have noticed I require more sleep or I feel loggy longer than in the past. I also do not heal from injuries as fast. Injuries, muscle pulls, etc. take a few weeks to disappear rather than a few days. I twisted my knee slipping on some ice last Feb and the pain and loss of pain free motion is finally nearly gone...five freakin' months...sheesh.
"Everything in Moderation" is probably the best advice I have ever heard.
Relax you have a long ride ahead of you.