First of all, biological age is not much of an indicator.
Secondly, I have been very strict over the past few years, looking for women that must be 1–4 years younger than me or something. I am almost 37, so this would generally mean women in their early 30s, who are often married or fresh out of a divorce. Or worse still, desperate for a husband. I have a friend who is a year older and will date anyone (or anything) between 16 and 66. Ok, so I’m not like him, but I realised that perhaps broadening my range to 20–45 or something may help. And of course after that you take into consideration real factors, such as maturity, compatibility and of course all the personality traits you are particularly looking for (plus looks and what have you).
My latest gf is 30, but the one before was 23. So I think there should be no rules.