I had a friend whose kids didn't come because of the age different between her sons and mine, and I said "I understand, that's fine". Then on the phone I'd mention "Joe's having a birthday party on __ " and leave it at that, and keep talking. Once she came without her kids, and we hung out. But after the first decline because of age differences, I never invited them again, because that is something that isn't going to change. A scheduling conflict, a sick kid, other reasons would have warranted another invitation the next time, but age differences will always be there, and aren't likely to even out until the kids are in their mid-20s, ya know? You can always meet up with your own friend another day.