What age do you let kids cross a busy road to go to a park alone?

Hey all, just looking for some perspective from other parents.

We live about six houses away from a park, but to get there you have to cross a pretty busy 55 mph road. There’s one crosswalk, and the park is also surrounded by a few large ponds.

The other day my wife was at the park with our kids and noticed their two kids (around 6 and maybe 3) playing there on their own. She had seen their mom walk them to the crosswalk earlier, but the kids stayed and played for over an hour without an adult there.

When my wife and our kids were leaving, she helped them cross back over since they were heading home at the same time.

What surprised us is that you can’t actually see the park from their house because there are trees and a hill in the way, so the kids are completely out of sight once they go over there.

So I’m curious — at what age would you feel comfortable letting your kids cross a busy road and go play at a park you can’t see from home? Is this pretty normal now, or would most parents wait until kids are older?