I have always wondered about this as well. Maybe as long as they quote months and not years they do not see them as children but babies. I don't know if it means anything but I have only seen mothers do this with their first born and those who have had more will not do this with the rest.

To me, it is because it is the 'norm'. Children under two are developing very rapidly and a month can make a huge difference. It is often easier to remember how many months your child is. Once a child is two, most stop going by month, when proclaiming 'age'.

I will also note that, at least in the US, children's clothing (up to size 2) is sized by 'months', as in 12 mos, 18 mos, etc.

I guess people think it makes their babies sound younger & cuter for a longer time. Personally, I think it's stupid and always told my kids' ages in years & months after they were past the 12 month mark.