It's not extremely common. Wrestlers do it and that's about it, until other athletes start doing it and it snowballs. It's not a good trend either, delaying your kids start of school because you want them to be more mature than the others. The remainder have the choice of also hindering their starts or being behind the increasingly higher number of kids doing this for an athletic or developmental advantage. I mentioned this to my colleagues here in Ohio, and they were dumbfounded that it's normal practice for wrestlers. One of them recalled that their state champion classmate was held back after I said it, and that it was a secret to everyone in the grade. One mentioned his kids team getting walloped by a team of handpicked older kids ruining the league for everyone. I'd personally be embarrassed being a 19 year old senior year. What the hell are they doing in high school still? Move on to college and stop playing with the kiddies.