It may be true that "As recently as 1999, it was commonly stated that babies could not feel pain until they were a year old ..." In fact, that is the actual statement in the Wikipedia article. It says nothing about "we" believing it.
It wouldn't be the first time I disagreed with a "commonly stated" belief, but I must admit I never heard this statement and a moment's reflection will show that this is a bold claim that would need some substantial proof before it could be believed.
In fact, if you go to the work cited in Wikipedia[1], you see:
> As late as 1999, it was stated that “[p]ain experience is placed at approximately 12 months of age” (66).
Checking that reference[2], we see in the abstract the statement
> Pain experience is placed at approximately 12 months of age, though this is within the context of a continuum of awareness rather than a straight ‘on‐off’ switch. The major moral implication of this stance is to place the burden of proof for analgesic use onto clinical measures, rather than relying upon the, so far, poorly supported assumption of pain awareness.
That last sentence indicates that the whole paper is written to argue against the assumption of pain awareness. On the basis of that snippet (I can't access the full paper), it looks like the assertion in Wikipedia is not valid, and your headline is even more misleading.
[1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3385812/
[2]: https://onlinelibrary.wiley.com/doi/abs/10.1111/1467-8519.00...
This is all MHO and anecdotal, so please don't think I'm trying to state anything as fact or an educated opinion.
I’ve also heard the hypothesis that babies will go into shock much earlier than adults and will therefore be silent, or start choking / having a seizure silently, during traumatic surgery.