I don't think that they're using age as a proxy for attractiveness.
Rather, I think they're trying to profit from an asymmetry that they believe exists: that there is a significant population of people over 28 who would really like to hook up with people under 28, and are willing to pay for the privilege.
They may also believe that they will be more attractive to their core demographic if there are fewer older people in their system.
They may even believe that they will be more attractive to their core demographic if their older demographic skews richer.
No idea whether they're right, but they presumably have a pretty good data-set to work from.
