My puppy Cleo is now 17 weeks old and she has been able to do sit for about a month, she also knows 'off' when she jumps on the sofa and hasnt been invited and 'no'. She can also do 'stay' when you put her dinner infront of her and wont eat it until she is told.
She is a little s*** the rest of the time though lol!
Start immediately - but with a bit of leeway and patience bearing in mind his/her age. They are never too young and labradors are very intelligent; they love to learn. Always done with reward for right rather than punishment for wrong, that's all.
About 4 weeks ago! Training starts the day you bring them home and never really stops. Everything from teaching them to be clean, to walking on a lead, coming when called, etc. all these things are easier to teach the younger you teach them.
Work on the principle that you never let the dog do anything wrong. For instance, don't wait till the dog pulls on the lead to teach it to walk properly on the lead.