We had Boo since he was a baby – and bonded our boys when Boo was probably around 3 months old, 1 month before his hormones kicked in. And unlike others’ experience, there was no impact to the bond between our boys after Boo matured or was neutered. Please note – this is not the norm.
This being said – we feel we made a mistake bonding them so early – because Boo is not well adjusted and very nervous now. We waited till he was large enough to defend himself during bonding, because Baxter was very aggressive towards him – but Boo has no confidence today. Had we waited till he was also neutered and larger than Baxter – he would probably be better adjusted today.
So I would advise caution – not just for the reasons others have experienced with bonds breaking when hormones kick in, but for the confidence level of the baby bun. It will also depend on how territorial/aggressive your 9 month old will be towards the baby. We were very surprised how aggressive Baxter was toward the baby. We decided to go with a baby, because Baxter was so intimidated by another larger bun we introduced him to. It’s impossible to tell how they’ll react 🙁