There are groups of what appear to be 8and under at the park often when I take DD. I don't really know how old, they seem really young, but probably are atleast in school since I didn't see them as much during the day. Or at all, hard to separate Saturdays from Wednesdays sometimes.
But we are on a base, so it's seen as 'safer' even though it's not really.
So I'd say it depends on where you live and on the kid. I'd prefer a friend with them at 5. but I'm not saying absolutely not as I know a few 5 yr olds who go to the parks around here with their older sibling (looks like 7 or so).
SAHM to Magnolia May (09/10) and Luke Russett (04/13) and wife and best friend to my airman.