I think it's interesting to look at why people have this idea though, because it's clearly false - first of all, today's teenagers are statistically much better behaved than their predecessors - they drink less, smoke less, have less sex, are less violent.
And there is good research showing that harsh punishment is no more effective at discouraging unwanted behaviour than mild punishment, and that positive approaches focusing on encouraging a wanted behaviour, alongside relationship, communication and skill building (so, modern "soft" parenting) is even more effective than any kind of punishment. And in fact associating some antisocial behaviours with harsh, physical punishment in particular.
So (putting aside the argument of whether or not entire groups believe this and who they might be) why does anyone think that children are more badly behaved today? Why do they believe that harsh punishment is so effective?
It's a common viewpoint - isn't it? There are supposedly records of people holding this view going back almost as far as we have records of anyone recording their opinion at all. So there must be something else going on.
My theory is that people are remembering how they themselves felt about rules and their own fear of harsh discipline and wrongly attribute their own good behaviour to fear of that discipline, and generalise that to their entire generation and/or all humans everywhere, and so they see bad behaviour happening and assume that the person must have felt enabled to do this because they were not scared of being punished for it.
I heard recently that there is some research which shows if you punish a particular behaviour harshly, people are more likely to do the behaviour after the punishment-rule is lifted compared to a condition where people are given only a mild punishment for a behaviour (they were more likely to self-impose a continued ban on the behaviour) - so this is interesting in this respect, because perhaps if you were raised with harsh discipline and believe that you only continue following those social rules out of fear of consequences, rather than having internalised and agreeing with the rules, you might feel like well, everyone wants to do that but we don't because it's not allowed - whereas someone who was raised with a more positive approach / mild punishment approach might have considered the rules for themselves and decided to keep following them because most social rules DO make logical sense. It's possible that some people who have experienced harsh discipline haven't had that experience and therefore doesn't understand that other people do.
Or it could be a reaction to changing norms around respect - if you look at older generations, the further you go back the more formalised this is - so when we were children we were expected to listen to adults and call teachers Mr X, Mrs Y (whereas family friends were Jane, Steve, maybe Aunty Jane etc.) But in my parents' generation, they were expected to completely defer to adults at all times, all adults were Mr X, Mrs Y, you did not speak unless spoken to by an adult, they may have been expected to stand when a teacher of particular rank (e.g. headteacher) entered the room. A generation before that, children would call all adults "sir" or you get to the point where children are "seen and not heard" etc.
So norms change and certainly there are rules from my childhood that I haven't bothered with for my children because I find them fussy and out of date. I do think some people notice this change in children's behaviour and rather than seeing it as a change in social norms and expectations, they see it as the children being incredibly disrespectful towards adults and the adults not doing anything about it.