"AGE (Older Drivers)
Age is no bar to the holding of a licence. DVLA requires confirmation at age of 70 that no medical disability is present, thereafter a 3-year licence is issued subject to satisfactory completion of medical questions on the application form. However, as ageing progresses, a driver or his/her relative(s) may be aware that the combination of progressive loss of memory, impairment in concentration and reaction time with possible loss of confidence, suggest consideration be given to cease driving. Physical frailty is not per se a bar to the holding of a licence."
So far as I am aware the only people who the DVLA will listen to are the person her or himself and their GP. Poor mobility or balance are not reasons to stop driving unless the balance affects sitting. Double vision may be but as i have said, its between the DVLA, the person and the GP.