What age should the foreskin of a child's penis go back over the glans?

Variable. Should be retractile by age 10 years. Definite abnormal if foreskin doesn't retract at puberty. Majority of boys have rettractictile foreskin between age 3 - 5 years. However foreskin should never be forceably retracted, this trauma can result in foreskin splitting. Such splits can result in scarring & phimosis, where foreskin is too tight, can not be retracted, & circumcision becomes necessary.