I joined a federal law enforcement agency at the age of 52, while remaining part-time in the Army Reserve as an Infantry soldier and officer for another six years. (I retired from the Army Reserve in 2018 with just under 42 years in the CAF) Now, 10 years later, I'm now the oldest full time instructor in my agency's defensive tactics team. Most of my peers are at least 25 years younger than I am. I ask for no quarter and none is given. But....
....my body knows how old I am, even if the mind denies it. I have several pre-existing injuries from the CAF and a few more from my current job. Somedays, my body tries to betray me.
Best advice I can give as an "old guy" is to look after yourself. Don't train harder than you are able to recover from. Avoid some of the dumbass things young troops like do to prove their toughness. If you get injured (and in the Combat Arms, most people eventually do get hurt), seek medical attention. Don't just "walk it off". And document every injury for pension purposes. Train smart and work smart and you'll have an exciting career in the Artillery!
I've said a number of times on this site - my sports med doc in school pointed out "After 40, your body doesn't forgive or forget what you've done to it previously, it doesn't fix itself as fast and it doesn't fix itself on your schedule". Just remember that...and what these guys say.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.