As a guy, physically I'd say from 16 to about 30. Psychologically things continue to get better - but when physical deterioration set in my early 50s - that ended my 'prime' which I would say 30-50.
For my wife who's 11 years my junior - I think it's 99% mental. During her child-rearing years she was not very interested in sex - even though she was not a working mother.
Luckily for me about 5 years ago she 'woke' up and things have gotten 1000x better. Unfortunately for me my prime had passed so I do the best I can to keep up.
We're now at the point that our sex-drives have converged to about 2-4x per week.