The guy described in the OP would have been my cue to say, "You should know better at your age!" I've yet to have this opportunity but I'm sure it won't be long!
Seriously, this is becoming 'mainstream' and not limited to low-end 'Gen Z' raised by parents who received insufficient discipline when they were kids (that's the problem). People of all ages and backgrounds go 'tiss tiss tiss' in the pub these days. If somebody tries to show me a 'video' I can only say "I'm sorry, but that's just a silly noise to me."
A recent memorable experience for me was not on a train, but on the bus. I heard the scratchy noise of a smartphone close behind me, and looking around I saw a completely respectable- and intelligent-looking bespectacled black girl (probably a student or an office worker). I wasn't sure what to say, it was as much as I could do to blurt out, "Is that you making that noise?" Thankfully she was respectable, etc. and turned it off, but it was the fact that she thought that it was OK to do this in the first place which shocked me.