50 is still young enough for singles if you work you way up to it, taking small steps to allow your body to adjust to the recovery needed, usually a full day of non strenous activity after a tennis day.
I know a few 70 year olds who play top of the heap 4.0 level singles, enough to bother most 4.5's. Of course, they didn't wear out their bodies in their youth, have no significant injuries, and know to rest and recover after hard days on the courts.
They also know when to let a ball go, that they would get to, but not do anything with, so they save their bodies.
You mind allows you to play singles forever, but your body limits your levels.