It's rare at this age for sure, but bit unheard of. I know someone who started at this age. My biggest advice is not to try to hide and control your stutter. That only layers in bad habits and shame. That is easier said than done but I would do my best to take it in stride. Sometimes you will stutter and sometimes you won't and the times you do, let it out. Holding back is what makes stuttering so difficult and painful.