I've read the other replies and my answer is pretty different. I think this has to do with a fact that up until 24 months many toddler-targeted things are spoken of in such fashion:

- Clothes are sized in months 0-3, 3-6 etc.. - During doctor visits you discuss developmental milestones expressed in months.

Etc. You get used to it, since at that age the development of a child is extremely condensed and years simply don't provide enough resolution.