Wait, wait that's how you get a tattoo on your butt? Not by making a decision at a young age of what career you think you'll do as an adult, but by some um... life change growth puberty thing? (I'm only here for Doctor Who, I don't really care MLP)
The "butt tattoo" or like it is called in the show "cutie mark" is something a pony gets when they discover their special talent. You could also probably say it is your destiny, the thing you are meant to do. So it can appear anytime as soon as a pony discovered their talent. It doesn't have to happen in the puberty. Technically it can even happen with adult ponys who haven't discovered their talent yet. When it comes to the life changing part, I would agree. If someone discovers that he is meant to be it will probably change his life.