Also, don't think that you can maintain the same high performance standard playing sports with 20 year olds. The Army plays a lot of sports and like
@Haggis mentioned you don't recover as fast and your body is not as supple as 20-30 year olds. I'm not saying don't play sports with them, but know you body and don't let your pride misjudge what you can actually do - you may need to ask for more reliefs but that's fine. It is really important to stretch for warm up and cool down.
The last time I played hockey with people in their 20s-30s, I was in my mid-50s and I was totally over confident with my abilities. I ended up injuring my knees - MCL and ACL tears. I was laid up with braces and crutches and was scolded by my wife with the never-ending "I told you so." As I still play sports, Voltaren and ibuprofen are my best friends - aided by the occaisional shot of a good scotch.
Use your life experience to your advantage. If you follow the advice from the guys above, you'll be fine.