The problem with that assertion is that it's so simplistic that it's meaningless.

Having more young people doesn't magically cause more violence. Those young people don't exist in a vaccuum. Adolescents generally don't have an income and are thus dependent on their families to provide for them.

Families having more children also means they have more expenses and need to allocate more time to parenting and "family time" for each child to still receive the same level of care. Additionally there are many economic and cultural factors why poorer people are more likely to have more children than wealthier families. Availability of contraception and abortion further exacerbates this. "More young people" generally means poor people having more kids. Even if the number of children increased equally across all economic categories, this would still mean more additional poor kids than more additonal rich kids because of the size of those groups (and because not-wealthy-but-not-poor families could end up poorer because of the additional expenses).

But sure, young people cause violence.

There's also the theory that humans caused the mammoths to go extinct not by hunting them to extinction directly, but by killing the adult males (because they have the largest tusks). Without a strong dominant presence to keep them in check, juveniles going through musth became so violent that they started killing each other and the females they pursued.

I find that sarcasm is always best when you can somewhat plausibly argue that it's serious.