it really depends on the person.. some people do it at a few months.. some people don't do it till after they're a year.. some people never cut their baby's coat.. and some just trim it.
I got Kayah cut when she was 6 months old...
I know that once their adult coat starts growing.. you're SUPPOSED to cut off all the puppy hair.. but again, It's all up to you and how you like their coat