I'm your age and see no reason to stop doing anything because of the date on my birth certificate. As long as you enjoy it, then why stop?
I think the only reason why you don't see a lot of 70 year old's snowboarding is that the sport just hasn't been around that long. You see tons of skiers, golfers, tennis players in that age group because they've been doing it all their lives, they love it and they keep doing it.
My 90 year old mom still plays 2 hours of tennis a day and that's a lot harder on the the joints.