Many airlines use the following CODPIE acronym for suitable ABP (Able bodied passengers) to be seated at self help exit rows;
C - Children (normally up to age 14)
O - Obese (require an extension seatbelt)
D - Deportees
P - Pregnant
I - Infirm
E - Elderly
Those categories should not be seated at a self help exit, but discretion should be used (ie plenty of senior travellers are perfectly suited to sit there, and an obese person might just be able to fasten the seat belt under their belly with considerable force (although seat belts at exit rows on my airline at least are shorter than standard seats!!)