One of my best fishing buddies is 72 and he can still hang in for a long day of fishing even in some pretty tough weather. I say put on a good PFD, fish with a buddy and plan to be at it for another 20 years at least.
I do not believe there is a length limit on crappie at Mark Twain. Fishing is very good there right now and catching a limit of 10 inch plus crappie should not be a problem.
I went to Mark Twain yesterday and fished from the bank with minnows for a couple of hours. Caught ten crappie. Shortest was 10" the longest was 12". Mostly females with eggs.