I played 11-a-side and 5-a-side until I was a shade over 45 before I finally gave it up. In the last 2 games I played I saw a lad break his leg in a veterans game and the week after, I nearly did mine. The bruise down my leg was the worst I'd seen. That was enough for me but I'd played in good teams for years. The lad who's leg got broke was unable to work for 3 months and that made me think a bit!
I've seen a LOT of players play into their 50s at a decent standard and in one 5-a-side game saw a 70 year old! He walked about and could control the ball and pass, but it shows that if you want to you can do it. You just need to remember you're body isn't what it was. It also makes you realise what an incredible achievement it is for a pro player to play into their late 30s!!
My advice if you want to play is:
1. ALWAYS, ALWAYS do a warm up. I used to turn up a good 15 minutes before KO and go through a stretching routine. It will warm your muscles up and help them not get torn etc. A muscle injury in your 40s will take about twice as long to recover as in your 20s and be more painful! This is probably my biggest advice - I was able to play most weeks and put most of this down to stretching. I've seen a surprisingly large number of lads pull muscles by doing shots on the keeper for warm ups!
2. Think about the game more. When I was 20 I would fly into tackles (cleanly) and wouldn't even think about backing out. When I got to my 40s, I realised I had to think more and avoid going for 50-50 balls. It's not worth it and again, keeps you injury-free. It also makes you a better player!
3. Don't be afraid to take a breather. It's better to play 30 minutes well by going in net for a break then flogging yourself into the ground for 60 minutes by not going in net! You won't enjoy it either. It goes without saying, but if you feel unwell etc, let someone know. If you play vets football you will be able to roll-on-roll-off which helps too!
4. Also, make sure you drink water regularly when you're playing. If you're feeling goosed, have a break and don't push yourself too hard.
5. Start jogging between games. You'd be surprised how much a run like 3k a few times a week will make a big difference to your health, weight and fitness. If you say play every Monday, have a couple of days off to recover then do a couple of light jogs to keep the muscles going. The lads who I know who did this were able to play well into their 50s at a decent standard.
6. Eat healthily. Eat well and you'll lose weight, be able to run more and enjoy the game more!
7. Don't be put off by age. I've played loads of games where lads in the late 50s who obviously aren't as fit as someone in the 40s but can still play, pass and shoot. You just have to remember you're not in your prime and no one else on the team will expect you to be able to sprint about! Also, a lot of lads in their 20s and 30s have a lot of respect for the older players too!
8. Do it gradually. In your first few games, take it easy. Build the muscles up, get used to the twisting, turning and running. Get used to running into space but don't blast it otherwise you'll not be able to play the week after as you'll be in muscle-pain-hell!
Hope this helps and good luck! ANY exercise is better than no exercise!