It's a really good question OP. I was thinking the same thing the other day. I was out with the dog one evening recently and my senses were 'alerted' by a loud, aggressive sounding male who I thought was arguing with someone.
I took a small peek round about trying not to make it look obvious and found it was a teenager walking along the pavement on his own but talking into his phone. I could hear his conversation because he was really loud. What I thought was a disagreement turned out to be him just talking to a friend arranging to meet up. However he was so loud and aggressive sounding with grunting noises, not looking where he was going or even it seemed aware of anyone around him.
I felt quite stressed and intimidated (fifties) thought I don't know why. Perhaps he was on drugs, perhaps he just had no manners but it did make me think how scary being an old woman was going to be with these types roaming about.
Another day I was out middle of day with dog and someone cycled by (a young man). It was not intimidating particularly although he was going quite fast past pedestrians which had I not seen him coming could have given me a fright. As he went past the smell of week was so strong and indeed he had a white stick hanging out his mouth so presumably he was smoking weed.
This is a middle class area considered decent to live in. Mostly private houses although with increasing social houses being built as part of the new build estates.
It did make me think how I was raised as a young person. I didn't have particularly good parents but manners were drilled into us as was respect for 'elders'. Giving up your seat on the bus to an old person or pregnant lady would have just been normal.
It's not just men though. i have seen young women behaving badly too.
My take on what is going wrong and it's a whole host of things.
40 Years ago if you were not too bright there was still quite a clear path for you to follow. You went into a more manual job (brickie, labourer, factory worker) and you got married and had kids. You could afford a house even on your wage.
So even though you hadn't done well at school you could still achieve things. Porn was top shelf magazines and the odd VHS video passed around which was generally very tame by modern standards with 'normal' sex. Men were expected to 'court' a woman and then get engaged and married so there was a process of waiting and wanting before you 'got'. You had to 'fit in' or you were very frowned on by society so everyone tried to follow the rules. Being unemployed was embarrassing and shameful. Hard work was rewarded and made you proud.
For those who were brighter or academic there path was the same but better job, better house etc.
Nowadays for those kids who have not done well at school there seems to be no jobs to go into, no apprenticeships (remember the YTS of the eighties), no clear cut path to follow. They can't afford a house so they stay at home. They feel shit so they take comfort online where they can watch awful violent porn or game constantly losing touch with real life. They feast on UPF so their brain is starved of nutrients meaning their health is poor. They don't think about getting married because how can they - they can't even afford a house so they feel trapped and hopeless. This turns to rage and anger. Combined with soft parenting and lack of being taught manners they don't know how to behave in the world.
it's more compllicated than that and I am generalising I know.
It's very, very scary.