It's probably hard to generalize,
Yes it is, but here goes
but do the older guys get any grief from younger recruits/instructors?
Sure, but so do younger guys.
If you mean are they picked on because they are older, No, not if people are doing their job. Age is irrelevant, fitness, ability to follow instruction, self discipline, dress and deportment and the ability to apply the training through job knowledge are.
Do many drop out of BQ before finishing?
No, in my experience as an instructor, 'older guys' are more likely to finish, unless they blow a knee
Any help/ideas/statistics appreciated.
Start running, everyday, whatever you can do, work your way up. Pushups, many pushups. Lose weight.
See GAP, above, read anything you can find, a lot of info here.