You can do it as early as you like, even 6 weeks! But, early on it's more of elimination communication and they need a lot of help from you to notice they need to potty and take them there. If you are in that habit it's almost seamless for them to go on their own when ready. There's a board on here under diapering about that.
Edit to add: usually if you begin early it's best to skip any reward or show of disappointment connected with pottying or missing, maybe a tiny bit of praise for going there but nothing big.