How come monta ellis started at age 20, but was drafted out of HS and immidiately played?

his birthday is october 26 so he was 19 when he graduated and turned 20 right before the season started. chances are he repeated a grade in elementary school. my daughters birthday is in october and if she doesent ever repeat any grades she will turn 18 at the beginning of her SR year. so he must have repeated a grade at some point before hs.