M.,
There's no set age, so try not to hold yourself to a certain date in your mind or compare him to other kids. Everyone does it at a different age, whether girls or boys. My son is almost 2 and he did the same thing your son is doing. Your son is just showing early signs of readiness, which is a great thing. He's identifying the action. Soon he'll recognize having the urge to go to the bathroom before he actually does it, and he'll let you know. Don't push him, but it's a great time to start talking to him about it all. When our son says pee or poo after going, we'll just say something like, "Yes, you peed in your diaper. Soon you will pee in the potty." And we just introduced it at first, but then had him sit on it at a regular time (for us it was right before bathtime). Sometimes he'd go, sometimes not, but if he did, we praised him a lot. For the last month, he had only been announcing the arrival of his pee or poo after it happened, but just recently, he started giving me a heads up and telling me in advance. It was then that I'd sometimes ask him if he had to go, and if he said yes, we went to the potty. He tells me every day now, and he goes at least a couple times a day, but not consistently enough to start training full force. He's afraid to poop on the potty still, but he will tell me 10 minutes before he's going to poo. He chickens out when we get to the potty and goes in his diaper. We're getting closer, but he's still very young and not quite ready. His twin sister has no interest at this age. His older sister was like him - interested early - and was fully potty trained & sleeping in underwear every night just after 2 1/2. Just start watching for signs. If you let it happen gradually without a lot of pressure, I think it's easier. It was for my daughter. Right now, I've noticed that my son will start pulling at his diaper with his hand before peeing. So that's when I ask him, and we make it to the potty more often than not. I know he's getting more and more ready because he can't stand being in a wet or dirty diaper anymore. If he wakes up from his nap wet, he wants his diaper off immediately. Take it easy, and good luck!