an alter of me.

Did you want "altar" by any chance? The sentence still parses but my brain kind of hung trying to make it fit with the context XD

some young people commented on social media how great it is that they had given the chance and exposure to the old guy on guitar. Of course, they were mocked mercilessly for their ignorance but for many of the younger generations, how would they know who he is?

Internet? :D

In fairness to them though that would require realising that the thing/idea one thinks is new might not be that new.

Also, the people doing the mocking could have tried informing them instead of being arseholes about it, but I don't know anything :)