My yekkishe cousins walked around with short hair from the age of two or something like that, but without a kappul and tzitzis until they were three.
I don’t understand exactly why they didn’t give a kappul and tzitzis before three if they cut the hair before three, but the kids looked like goyim 🙂
I personally only had a hair cut at three because i am litvish on the maternal side and my grandad is makpid but apparently there is no mekor for it at all.