We have a fenced yard. My kids were allowed to be out pretty young but I did a lot of gardening when they were young so it was pretty rare they were out there long without me, maybe 2-2.5? I also have a row of widows that looks out on our postage sized lot.
My eldest (about to be 18) still plays outside if she has time - lawn games trampoline or entertains the neighbors occasionally.
My younger two (16.5 and 12.5) are out every single nice day
We’ve always encouraged them as much as possible and changed w their ages (purchased lawn games chalk anything to get them out