DS1 was outside from about 12 months in a fully fenced yard. I checked every few minutes and could see and hear him from the house. He learned to negotiate our deck steps very early on and was confident with stairs as soon as he could walk. DS2 is 15 months and is always outside with now 3 year old DS1, has been for a couple of months. Again, I check every few minutes or sit on the deck. We've moved and out yard is much larger, has a cubby which has steps, lots of garden beds to hide in and a swing set. They both have a ball. I'm more vigilant now than I was with DS1 but only because we now have a puppy. But I still let them free range the back yard a lot. I'm sure a lot of parents would be a bit horrified about it but meh, they're fine. They've both had tumbles, falls, grazes, step issues etc and I've heard or seen every single cry and been by their side a flash if it's a real cry. The crocodile tear cries I leave until they come to me. They're both happy, healthy boys.