This actually happened to me recently with my twin girls.  (In my world, I was watching cartoons though.)   All my girls love back rubs, but only for awhile.  Not HOW LONG you do them, that can go on forever and it’s hard to find a mutually agreed upon stopping point.  No, I mean, at what AGE they are when they are no longer interested in them- at least from me.  It seems to time out to be about the same time there is a bra strap back there.  Not that it has anything to do with said strap, but it has everything to do with the “don’t touch me” emotions that go on at that age.  Also, I don’t think I’ve ever given one of my twins a back rub without the other one popping up wanting one too.  Because of that, they will never know what a two handed back rub actually feels like, the poor children.