Ah yes the classic (your age / 2) + 7.
But that is more a joke than real advice.
I always say to my friends:
- youngest: legal age. 18 in my country. Not age of consent. Legal adult age.
- oldest: as old as you can tolerate (i.e. not be grossed out :-0 )
The age of the person is not that important. I dated a women 13 years older than me, and currently I am with a women 14 years younger. The number is not what is important here. How you feel about the person is. Sure some will judge, but what is more important? Being happy or miserable based on someone else's criteria?
The older you get, the less important it is since the differences tend to flat out as an adult. Obviously when you are 20, 40 seems real old. But as you get older, 40 is fine. There are just some details you must know. Ex. certain ages come with common desires. A 35 year old single women will have her biological clock running to the max! Younger will be more about party and establishing a career, for example.