What age do you start teaching about "stranger-danger"?

My 3 year-old is very friendly and sociable. He waves and says Hello to random people, sometimes try to start a conversation and I must say it makes me feel uncomfortable sometimes. I wish he wouldn't make himself that noticed but his behaviour is somehow reinforced by people who talk back to him. Obviously, I don't mind so much the supermarket lady but he also proudly showed off his toy the other day to some kind of junkie who was "recovering" at the same bus-stop... Embaring. Is it too early to tell him not to talk to strangers and what reason shall I give? Remember, he's just 3. Thank you for your answers.