This is an excellent point, and I wish it weren't buried so deep in replies. If dating apps make it easier for "desirable" individuals of both sexes to have lots of partners, then the effect you describe is amplified. That is, being "desirable" is partly about having money to pay for a nice date, a car to pick up your date with, etc. A 21-year-old is less likely to have these things than a 32-year-old (who is still young enough to be desirable to many women in their 20s).