I started at 36, and it's hard! Starting later than that is admirable. But it really depends on the school, too. I know in our school, there are quite a few "seniors", as well as young guys. It's a good mix, and while there is always a lot of encouragement to keep going and not give up, there's no shame in tapping because you just can't breathe anymore. Regarding sparring with a plan, I think that's a great idea. I'm going to start thinking about that. I do a little of that already, in that I have a few techniques that I look for and work on in every class.