I bought the 12-18 size for my 14 month old, and the fit ok.  They are a little snug, which is strange because my son is neither tall nor fat for his age.  However, the feet are NOT non-slip.  The "traction" on them is just a bunch of thread sewn into a kinda polka-dot pattern.  My son was slipping and sliding all over our kitchen floor.  Not safe.