I have trouble finding the 24 month size in local stores, so rely on Amazon for those.

After washing and drying, these Gerber onesies usually shrink somewhat.  So, depending on the size of the baby, they are good from about 18-24 months of age.

I find the onesies key for layering in the winter.  And in the warmer weather, a onesie and a diaper is a go-to outfit for toddling around the house.