Foreskin
In any case, sometime between infancy and the onset of puberty, the foreskin separates from the glans of the penis and can slide back and forth over the glans of the penis. Often this happens sometime after age five. This happens because of normal erections during childhood. Erections stretch the foreskin. There is no "right" age for this to happen. Each child is different. 99% of foreskins are retractable by puberty. Once the foreskin becomes retractable, usually it will automatically retract with an erection. Alternatively, a male (or his sexual partner) may slide the foreskin back and forth over the glans of the penis to cause an erection.