I think it depends more on the mom and dad than the child. If the parents are light sleepers and make a hard and fast rule about being awake when the children are awake, less is needed. If the parents teach the child from a very young age to be in a certain place, where the parents say to be, and to have bounderies, less is needed. So many parents give their children the run of the house. If they get tired of chasing them and teaching them to stay out of things, they either child-proof, or they put the child in a crib. Either of these things will keep the baby safe or keep the things safe from baby. But neither of these things teach the baby very much.
I don't believe that any child should ever be alone unless the child is in a room with very few hazards, they are old enough to know and trustworthy enough to obey the rules. Even when you trust a child, never go more then 5-10 minutes without poking your head in. Sometimes poke your head in the room every 3-5 minutes. In other words, the child needs to know you are NEVER away, even when you are technically away. It's not unlikely for me to run to the sink and wash 2-3 dishes and rinse and place them in the drainer, walk around the house and check the various computers, play stations, and desks to see that the children are doing what they are supposed to, go back to a few more dishes, go to the bathroom, check on the kids, back to the dishes, sweep some floors, check on the kids,... you get the picture.
I do almost ZERO proofing. It makes me CRAZY that my toddlers bring me the electrical socket coverings. The only way to keep a child safe is to SUPERVISE. I think that child-proofing often allows some parents to be laxadazy in their parenting.