Oldest I know of is 57, but I'm sure that's not the record. I'd planned to do it last year with a friend and his 65-year-old wife, but we had to cancel. If the RAAM (Ride Across America) organization is still around, they might have records, but I haven't seen anything from them in several years.
FWIW, I know an 84-year-old man who rode from Reno to Salt Lake City a couple of years ago, 500+ miles, and said he could have kept going..