I am a 25 years old woman dating someone in his 40s. The relationship is great and I honestly feel like he makes me feel better than ever. However, the influence of your social circle is a huuuge obstacle.

Your friends outright make fun of you if you date someone so old. You lose your status in pretty much all female circles. And yes, this is important to women. Very few women don't care about the opinion of their social circle and those typically have some mental issues. (remember that it is a natural instinct for women to want to be accepted)

So no. 20 years olds are not usually dating significantly older men. I know it because I live it. All of my friends are dating men around their age or a little older.