My husband was out a few days ago picking weeds, when a lady asked him if he was collecting for tortoises. She then went on to tell him she has a tortoise which she has had since she was 9 years old - she's now 64!!
She has given us a formula she uses to judge if well enough to hibernate, not sure if any of you have come across it:
measure weight in gms
measure length in cms
cube length
divide weight by cubed length
if answer is 2 or above it ok to hibernate.

Ive heard of similar forumulas, so yes at the age of 55+ she must be doing something correct