I think that for whatever reason, Rumple had to accept the fact that he was going to lose some time with Bae in regards to that twenty eight years when the curse was in effect (maybe it had something to do with the way the magic worked, or maybe the curse was the only way he could get to this world. he did put in the safety valve regarding Emma to break it after all). If the curse and Bae came through at the same time, then Bae would keep ageing like August and Emma because he wasn’t IN storybrook, even if he was originally of that world. I think that after centuries of Rumple desperate for a do-over, he would jump at the chance to have his son back at all, even if that meant he was almost thirty years older then the last time they saw each other.