We reinforce them all the time, oftentimes without knowing... A baby cries, in comes mama, that's reinforcement... Reinforcement is but that feedback one gets in the communication process. We always reinforce... sometimes reinforcement could promote what we define as negative behavior.
In short, in my opinion, there is no set age to consciously start reinforcing behavior.
Start at day zero, go from there.
For example, I never came rushing to my children if they threw a tantrum, for example. I always let them know that it was not them, but me, who was in control. Their lack of control was not going to control the situation.
Another example, public behavior and private behavior. Some parents pretend their children to have certain behavior outside the house, when they actually allow or reinforce same behavior in the privacy of their home. This is very unfair to the child as their souls are candid enough not to know the difference between public and private. So, make sure you let your children know how you want them to behave in public by promoting such behavior wherever you are, albeit home or at the mall.
Letting them know has nothing to do with verbal communication. Everything communicates. And certainly when it comes to discipline and reinforcement, non-verbals work wonders, especially if you are being consistent with your discipline method.
I could go on, life is not a cookie cutter. I have a 20 year old man that absolutely by now flies solo :-) and two reinforcable little ones, well, not that little, of 10 and 8. So, I've been through every stage.
In essence, if you mean it to convey your message they will certainly get it. If you're playing they will get your drift to... and you'll be the one reinforced, it doesn't matter if he's two or 20.