Plus, of course, it's still not that rare for people elsewhere in "IT" to switch over to software development at some point. They may actually be willing to take a salary cut and work for entry-level pay if that's what it takes to make the switch.
There are many reasons why pay alone doesn't "keep the old guys away", and some companies really do only want young workers. They tend to be very exploitative companies, however, banking on someone in their first job not recognizing how badly they're being used. Age discrimination may well be low on the list of sins for some of these companies.
This pretty much says it all right here.
They might as well advertise for "Naive, spinless young suckers who'll do anything for a buck."