I am about the same age as your mom. Been around horses my whole life, trained them, etc. I decided to get another horse for myself and bought a youngester with good training, gentle, etc. I rode him for awhile, did more schooling, but since it was a youngster, it was not a really enjoyable time, youngsters just do dumb stuff. I traded him for a now 9 year old mare that is absolutely perfect. She is young enough to ride for hours and hours, does whatever I want, yet doesn't have a kid mentality and has been there done that . I can lay her off all winter, then climb back on and go for a ride. Trails, arena, she does it all and is mature enough to have the "antics" of a kid gone.
My vote is for an experienced horse that she can have fun with and enjoy and not worry about being a silly horse