I gotta agree with the age related factors of slowing a guy down and causing myself to re-think trying things that 15 years ago I wouldn't even have slowed down for. At 45 you heal a lot slower. I have a career that requires me to be in good physical condition. If I hurt myself sledding I'm screwed work wise and therefore financially.
If I get out with guys that are better sledders than myself I may attempt a bit more because I know they are there to help me out if I get badly buried...but I know my limitations and at this point in my life and I listen more to common sense than I used to. I have nothing to prove to anyone.
As far as practicing goes,
I agree with Rotax that my practice for sledding is done in the gym. Keeping in shape makes a world of difference of how your entire day goes.