There are fifth and sixth graders at my daughter's school -- kids who are 11 and 12 -- whom I have mistaken, inside the school building, for teenagers from the high school next door. I didn't mistake them for middle schoolers; I thought they were high schoolers! I thought maybe they were in the elementary school doing service hours or something like that. Then on other days I'd see them with certain teachers and classes and realize: They are elementary students, but very tall and/or very mature-looking.
So I wouldn't necessarily assume that a child who appears to be 14 is actually 14, after those experiences.
One other small thing to keep in the back of your mind: We don't always know every child's specific circumstances. While some of the kids you're seeing may indeed be 14 and just running wild where they shouldn't be, others could be 14 or 15 (or younger, and big) and developmentally disabled in ways that do not show outwardly, but which make it more appropriate for them to be at a younger kids' playground. Yes, they may be physically large but developmentally still "playground age" and their parents are desperately wanting to get them out of the house and get them to burn off some energy. I've encountered kids who seemed to old or big to be somewhere or doing some activity that seemed too "young" for them and later I observed and realized they might have developmental issues that meant they were still enjoying the younger stuff-- and needed the exercise.
But in those cases parents or caregivers would be with them. Where it's very clearly older kids with zero adults, I would bet that loudly saying, "Hey, where are your parents? Are you older than 12?" might get them heading off the playground pretty soon.