I bought these for my 10mo son who is bigger than the average (90-95th percentile in length, 75th in weight) and grew out of Adidas Original (not NEO) size 3 when he was about 5-6mo. He has size 5 shoes from other brands but they are all too big so I bought size 4 this time, but they were too small so I exchanged to size 5, which are perfect on his feet. I think NEO's sizing is a little smaller than other brands.