It's not like 50-year-old dicks are covered in mold or anything. :P Generally by the time I do anything sexual, I've been talking with the client for awhile, making out, etc., and I'm not physically turned off by wrinkles or white hair. (I don't date older guys because I dislike the difference in life experience, not because old = ugly.) Also, pretty much all of the old guys are waaaaaay into giving me oral, so after half an hour of getting eaten out, I'm pretty turned on.
You know, I've never had a really hideous client. No Quasimodo types, anyway. Most of the unattractive ones were so eager to be friendly and nice to me (like the aforementioned fat guy) that I end up liking them.