I don't think it's too old, but old is a relative thing.
If the guy is training a bunch of pros and/or pro prospects and he thinks you're going to be more of a hinderance than anything else, then I can KIND OF see his point.
If it's a bunch of kids, like 6-13 years old and you want to spar with them, then yeah, you're too old
If he's not sure if you can handle it from a health perspective, then ask if you need a physician's clearance (but I haven't seen this for someone who's 50).
If he's just a doucher, then find a better place. Don't feel bad about not sparring being a deal breaker; it would be for me and a lot of people here too.