They only baby gnomes I've seen are Mysterious Mr Gnomes.
At least some of the other gnomes have 5 life stages. I found this out by using a cheat and clicking on the gnomes I had in game and found a pie menu that let me choose a life stage ranging from toddler to elder.
I discovered I could stop my gnomes from dying by changing them back to toddler when they aged up.
The only gnomes that seem to me to change their appearance for each life state are Mysterious Mr Gnomes and the Ghost gnomes.
The Ghost gnomes change colour.
Mysterious Mr actually looks like a baby with a nappy etc.
I forget the cheat I used to discover this thing about life stages, but it is in this thread somewhere as I do remember telling people about it when I found out and it won't be very far back so if you want to look for it I'd start at this end of the thread and work backwards.
I've not been playing the game much for a few weeks as I'm busy with another project at the moment.