I've search all over, but does anyone know the world's record for a coast to coast ride by a female cyclist. Not in time, but in age. Who was the oldest?

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..

I don't know of any reason why anyone who rides a lot couldn't do it.

As for racing, Bonnie Allison covered the 3000 miles in 1992 in just over 14 days. She was 50 at the time,
My friend David Jones did the race in just under 12 days a couple years ago when he was 62. He's at it again right now.