We live in MI where the weather is drastically different between winter and summer months.
DS1 is 3.5 and is solidly in size 4 and some 5s for spring/summer clothing. I'm not sure if some of the size 4 clothing I have for fall will last all of next season. I will probably have to supplement.
DS2 is 8 months and solidly in 12 months and starting to wear 12-18 months clothing. Not sure what he'll be wearing in the fall/winter! He's a tall baby!