Think about this anatomically.
A Woman is able to bear children from (about) age 12 and then Safely, bear children until about age 30. between these ages, the likelihood for birth defects are about 2%. Not very likely.
After the age of 30, the birth defect rate increases substantially.
Men however, are able to produce healthy sperm until just before death.
So that's a biological reason, but now for a more subtle approach to your question.
A mature man will hold knowledge, wisdom, a steady job, and the know-how to run a household (bills, mortgage, etc). Combine that with the fact that a lot of women crave stability in a relationship. They crave a future with someone they love. and mature men feel that they can provide that for a woman. This boots the mature mans' confidence to approach younger women and they think that they have a chance with them. Mature men feel less like the creepy uncle who had too much to drink at the christmas party, and feel more like Hugh Heffner.
So there you go. Creepy old men, feel like they have a chance with you both biologically (to preserve the species) and Psychologically (to preserve you).