I figure that the assumption being made is that the alt-right social media pipeline is generally accepted to refer to the online ecosystem that was created to court impressionable young boys from the ages of 12-25 into the rightwing and then extreme rightwing ecosystem. And though Men's Rights Activists is more broad and could be considered a choice, it is the younger demographic that are the ones "falling in", and that's where I imagine Vaush clarified that with saying "12 y.o.s".

Older folks entering involuntarily, would have been through other media like Rush Limbaugh-style AM radio, Fox News or other Republican campaigns directly like Project Redmap in 2010, then those Republican groups shifted even further right over the last 15 years.

Perhaps the correlation that 12 year olds are the ones falling in even if others are doing it by choice, should have been explained by Vaush before jumping directly to the conclusion.

Sidenote: The problem is caused by the difficulty to have nuanced arguments within 150 characters. (The first two Twitter screencaps)