This happened to my friends daughter. She was with her friends and an older man (~65ish) approached them and told them they were too old to be in the playground. He demanded that they leave before they broke equipment 'with their weight'. He was very persistent and a few of the girls were upset.
The playground was very quiet at the time - just a few other kids.
None of the parents of the 12 year olds were close by - a few of the girls phoned their Mums who arrived quickly but he was gone by then. He didn't have a child himself which the girls noted and thought was odd.
My friend says her daughter won't go there anymore. I think this is a pity as it was a safe area (I thought!) for them to hang out. If there had been more children in the playground they would have moved to the side but the girls said nobody was waiting to go on equipment. The equipment is very sturdy - I've seen adults sitting on swings in there before!
What should they have done?