Old thread to bump...
In the Cal Ripken Senior League, 11 and 12 y/o pitching distance is 50' and the bases are 70' apart. It's perfect for that age.
The Junior Ripken is 46/60, I believe. I think it's that way because the mounds are constructed for 2 pitching plates and if the shorter distance was 45', then the older kids would be striding right to the mounting hardware of the shorter distance.
Either way 45 or 46 feet is just about right for 10U age group. 11 and 12 y/o throwing from 45 feet is just too tough on the hitters if the pitcher is throwing gas.