Wow, OK…
Do not use the Interaction called “Raising a Newborn” when interacting with a newborn.
It removes the stage and dumps you straight into toddler.
I did that interaction seeing that it was new, and even though the baby being 2 hours old in-game he aged right away >.<
I didn’t think to warn you, but it happened to me too 
That’s actually cool for me, I never liked this stage anyway 
FYI.
I had ageing set to OFF (for all zois and npcs) and used the “Raising a Newborn”. It did not age the baby.
Nice to know though that with ageing ON, parents can actually “raise” their kids by picking them up 
Just so, I hope picking up your loved one using one of the romantic interactions doesn’t age them to a senior 