A phrase, often preceded with the word 'Oh', said by one who considers themselves above the person they're telling it to. And usually, it's true. People all develop through similar stages, and so when you see someone exhibiting symptoms of being in a stage you've already been through, it's only natural to want to shake them and open their eyes to their own immaturity. Unfortunately, people never learn things the easy way, so you're better off doing nothing. People are usually happy to be and act stupid anyway.