The way I was raised: when someone greets you, you greet them back. Even if you don't know them.
In urban areas with lots of people, you don't greet strangers in the street, but if they greet you, you greet them back.
The reason those kids don't greet you is... irrelevant IMO. It's their thing. Bothering to get them to greet you back - the whole topic on that... that's a bit creepy IMO. No offence meant, really.
For people I regularly pass by on my commute, I stick to a nod. If they don't reply, I stop doing even that and just ride on.