I would say that the population is, on average, much older and has more to lose. Fortysomethings do not regularly engage in street chaos. This is even more visible in Europe, even in formerly violent places like the Balkans or Northern Ireland.
Once the share of the young hotheads in the population drops, you have much smaller probability of really serious riots.
Ironically, we are studying whether lead caused cohesion problems for the Roman Empire [1] [2], so it isn't as if we didn't suspect lead before to cause civilization-scale problems. And while the modern drop in blood lead levels is big in the developed world [3], it is still orders of magnitude greater than pre-industrial levels.
[1] https://www.forbes.com/sites/kristinakillgrove/2019/11/29/ar...
[2] https://penelope.uchicago.edu/~grout/encyclopaedia_romana/wi...
[3] https://www.who.int/docstore/bulletin/pdf/2000/issue9/bu0686...