Most of the attendance of protests in the US I've been seeing lately have had older crowds. However, some of the ones I know have been organized by younger people have had rather good turnout with entirely younger crowds. Don't think it's a lack of interest of younger people just they're not hearing about it existing as much

A good chunk of this boils down to use of entirely different platforms. For instance, people getting the word primarily on on Facebook isn't seen by most of the <40 crowd