>>82190240
The major difference is that back then, even under trump, there was censorship against the far-right. This time around the far-right is being allowed to spread. To say the alt-right had no impact on young people just isn't true, but it was limited because of the greater degree of censorship.
"24 million Americans share beliefs with the alt-right":

Another report from the same era says 11 million:
https://www.vox.com/2018/8/10/17670992/study-white-americans-alt-right-racism-white-nationalists
https://archive.is/ME7LJ

As you can see from the image and the other sources, far-right attitudes peaked when the alt-right was as its height before censorship killed it. It's rising again as you can also see because the right wing is making a comeback as censorship is no longer an issue.
https://pro.morningconsult.com/analyst-reports/gen-z-politics

The alt right undoubtedly had a great impact, and most sources agree that, at that time (2015 - 2018), the right wing grew. Again, the rolling back of censorship is why the far right is now growing again.