I think there are many factors besides age and body,,
when you go from looking good, to looking like someone who is in denial about age,,it might be time for a one piece
if you have a pre-teen or teen daughter, and you will have more guys looking at you than her,,it might be time for a one piece....
if it's a family vacation and you want to look like the mother , rather than the nanny...LOL it might be time for a one piece...
if you put a bikini on and look like you are ready for the Springer show..it might be time for a one piece......LOL