We used to play on playgrounds sometimes when I was in college, but I seem to remember that around pre-teen age--maybe 11-12--going through a stage where we wanted to be frightfully grown up and not be associated with anything that was for "little kids." I seem to recall some older kids hanging out at the local parks and bullying kids at the playgrounds when was 8-9 too.
I do see sometimes kids out at the local middle school tossing balls around, riding skateboards etc. That's not really a playground, though. When I think of playgrounds, I think of special play areas with those forts, swing sets, climbing bars, merry go rounds etc. Sometimes kids segue from the playgrounds to other facilities at the local park, like sports areas, skate parks etc.
When I walk at the local park with a play area, I see mostly parents with younger kids, like maybe 8 and under, and few older, unescorted kids. I can't help wondering whether or not kids stop playing on playgrounds earlier now, because they spend more time indoors, playing video games, and online or with their phones. Plus, in middle-class suburbia at least, kids are shunted into a host of structured activities (youth sports and so on) after school and on weekends, so they don't have a lot of time to just hang out somewhere with their friends.
I'm not a parent, though. People with kids can probably provide the most current feedback about the age when theirs did, but it might vary with the kids in question and with geography. Kids in small towns may be allowed out without parental supervision at a younger age, for instance, and there may be fewer structured activities taking their time up on weekends and after school (and less to do in general). They may hang out on playgrounds at a later age.