At what age can you completely pull back the foreskin?

I have always been able to retract my foreskin as long as I can remember and so have my children, We have never been shy to encourage gently loosening this part of the anatomy in our family, however normally there seems to be no hard and fast rule and for some this does not happen until they are well into their teens. This would be especialy prevalent in situations where boys are made to feel bad about handling their genitalia.