The rule I have used is that the you take the age if the older one, divide by 2, and add 7 to get the minimum age for the younger partner. So a 50 year-old should go no younger than 32; a 30 year-old could go as young as 22; and an 18 year-old can go to 16. At 53, I met my current wife, 36; we are 8 years happy.