How old is too old to start snowboarding/skiing/similar sports, for pleasure and exercise?

Acquaintances who were avid expert skiers from childhood seem to give it up in their 80s, so all this "never too old to take it up", let alone keep doing it, is a real stretch. I once taught an adventurous guy in his 80s to ski. He could definitely do the movements, and learned the essentials, but he and I were nervous that he'd crash hard at some point and get hurt. I don't think he kept it up. The issue is bone density, and elasticity of connective tissue, not strength nor state of mind. I'd say 70s might be the upper limit for taking up alpine skiing, 60s for snowboarding, and maybe 80s for flat ground cross country. This assumes a very high degree of fitness and athleticism.

I have a friend who gave a first time lesson to a woman on her 80th birthday.  They just made sure that that took it slow, and took lots of breaks.  I know other adults that learned to snowboard in the 60's.  I don't think you're ever too old to learn, but you made need to adjust your pace and your expectations to match your current fitness level.  If you focus on having fun, it won't really matter how much "progress" you make.

You are never too old, I've seen "grays on trays" do very well. To avoid the most common snowboarding injury (broken wrist), I recommend using gloves/mittens with built-in wrist protection or using skate-boarding wrist protectors. I started in my 30's and I would have broken bones without them.

I know a woman who started skiing in her 50's and continued to ski until she's in her 70's, so definitely no age limit!

No age limit, but the more fit in general you can make yourself, the better the whole experience will be. There is a minimum level of fitness that exists for any new sport, and is dependent on your age. If you are a 50 Y/O who eats well and exercises vigorously several times per week with not specific impediments of previous injuries that might cause concern like bad knees or back/neck issues or whatever, you should be fine. Take lessons from a professional, take it slow, and don't let your pride make you do something that takes a long time to heal from. I'm 53 and have been snowboarding for 26 years on & off. If I stay mellow and limit my runs to big carves and stay mostly off the terrain park, a day on the slopes is no more tiring or painful at the end of the day than a day of any other physical activity like hiking or riding a bike or whatever.

Yeah, just like me, I don't ride to be a pro, I just do it for fun, exercise, breathing fresh air, and bonding with nature. Maybe forties will be the oldest age of learning snowboard/skiing for the first time. Yeah, injuries has greater risk if you start at that old. It's better that you at least know how to ride, so you'll get used to it and also to avoid later serious injuries.

Just don't try wakeboarding.  Wakeboarding should be reserved for those of an age that bounce well.