Slate has pointed out that Leonardo DiCaprio has a very specific type of girlfriend: they always fall within the age range of 20-25. And they also have to be blonde supermodels.
DiCaprio broke up with his most recent girlfriend, Kelly Rohrbach, who is set to star with The Rock in the movie adaptation of Baywatch, in December. She's also 25, which means she aged out for Leo. For the record, although it appears he won't sleep with women older than 25, he did sleep in an animal carcass for his new film The Revenant.
Apparently, all of his girlfriends have fallen within this age range, and their relationships with him end before any of them exceed the age of 25. George Clooney waited until he was 53 to get married, and he settled down with a nice lawyer. So it looks like Leo has at least another 12 years of living the dream until he freaks out and actually wants to settle down.
That may be why he dates younger women; he's not yet ready to settle down. Or it could just be because he's following the playbook for what to do as a super famous male movie star. Until then, it seems likely he'll find more than a few women willing to date him.