Biologically speaking, this is due to the fact that women reach peak fertility with the maximum number of eggs that she'll have in her life, at 18. However even with this fact in mind, men are often shamed for this sexual attraction. Considering that men's main biological function in life is to spread their genetics to the next generation to ensure that hundreds and thousands of years of your ancestor's successfull genetic survival does not die out in an instant, it makes sense that women of the most fertile age of 18 would be most attractive to men as they would best ensure the survival of these genetics.