At 33 I'm surprised that you are particularly concerned about this. I'm 30, working in Neuro which is probably the physically most demanding speciality and I've no plans on stopping soon...anyhow that's a different matter. I have worked with 50 year olds still working in Neuro though she is an exception rather than the rule.

Women's health is probably a good speciality for physically less demanding work. Or hand therapy? I'm not sure on the latter but I would guess it is largely done in sitting.