I do think that 20 and 32 is a bigger difference than 29 and 40 (plus, he was your boss right so you two have been through a bit together). I'm curious as to where your ex met his current. Bar is out, church singles group is probably out, is she an undergrad and he a grad student at the same school?
One that I've noticed in my friends is often after a long relationship and a break up one person will date someone totally and completely opposite of their ex...and often be wonderfully happy with that person. I brought that up because of the kids issue. You obviviously know Aaron better than I do, but maybe he's looking for something a bit different than what he wanted when you two were together?