Why I am afraid of youth now

So it has come to this...
I am afraid of 'the youth'. Not in an 'oh they're all so undisciplined, how dare they slouch about the streets on their alcopops and their internets' kind of way. But in an 'oh god how did I get this much older than them without commanding any of their respect' kind of way?
I refer to a specific incident which made me fume at how helpless I felt when I was in no way disadvantaged. I was out with two friends in Greenwich, we were walking through a park. There was a group of maybe eight teens, perhaps seventeen years old. Not that many, and not too threatening looking. But I had seen them pulling branches down off the trees and tearing up their leaves, so I was a bit disapproving. Then as we continued to walk out of the park, a stone came skittering past our feet. I turned around and, seeing the bunch of teens still behind us, gave them a scowl for throwing stones, which I think we all agree is usually a bad idea. The scowl, it seems, did not go unnoticed, and the next thing I knew, a large seed pod had smacked me in the back of the head. I immediately spun round and said 'please don't do that, you twats.' Which may be one of the strangest and sadly, bravest things I have done (more recently, I jumped off of a waterfall.) At least they hadn't thrown a stone at me. Suffice to say, nothing bad actually happened, and thankfully, one of the friends I was with is a teacher, so was more than adapted to getting rid of little shits. I suppose the feeling that affected me most of all was 'why does this happen?'
Why are we never too old to feel threatened or bullied when in reality, I'm an adult. At the very least, I can call the police. Or perhaps their parents. But these young people thought that it was okay to throw something at my head. Profoundly disturbing.
Next week: why I am afraid of the elderly.