Baby clothing sizes are based on months, based on the sizes of average babies at certain ages; keep in mind that some babies will be bigger or smaller than average. If the size only gives one month instead of a range, remember that it will be the same size as the higher end of the range; for example, 3-6 months clothing is the same size as 6 month clothing.
Toddler clothing sizes are based on children’s ages in years, and you’ll probably only need new clothes once a year at this stage. You can tell it’s a toddler size by the T after the number.
Both girls’ clothing and boys’ clothing follow the same sizing parameters from ages 4 to 6, then split, as the sizes no longer coordinate with age.