"An ox forgot [how it was] when it was a calf" is used to describe a person that has no mercy/is harsh on someone younger when he makes a mistake. Generally speaking, the older one pretends to be Mr. Know-it-all despite the fact that he also made the same mistake, or generally made mistakes, when he was younger. How can we say the same thing in English?
EDIT
Because there is no direct translation: if I said this idiom to you (that has never heard it before) would you get the meaning?