I have been out of the dating scene for over 3 years, as I've been in a committed relationship and now have a child, but when I was last single, I will admit that I did mess around with some girls that were substantially younger than me. However, I never wanted a relationship with any of those girls, and I made that clear to them from the start. They liked me because I was older/more mature/smarter than all of the young punks they usually dated. However, I knew if we attempted a relationship, the age difference would take its toll, and it really would have no realistic future. Even a "mature" 25 year old is almost always in a different place in life than a 40 year old, and it will greatly affect any relationship once the initial luster wears off.
My girlfriend is 2 years younger than me. My previous girlfriend was 9 months younger than me. The one before that was 2 years younger, and the one before that was also 2 years younger. I think it's best that way.