A baby can crawl in random directions. Of course I'll protect him from crawling off a ledge. But when he has the capacity to understand and judge I coddle as little as possible and expose as much possible. 5-6 is around the right time.

At 3 I don't let him run across the street. At 5 I tell him to look both ways and I tell him what happens to someone who doesn't, then I let him play outside.