Styxhexenhammer666 and the mystery of David Hogg's age

Recently, Styxhexenhammer666 made a video about David Hogg getting accepted to Harvard.  And there was something odd about the start of his video. 

Here's my transcript:   

Alright everyone, unfortunately time to make another video about David Hogg.  See now he's like 20 now so you can actually use his name.  Before for some reason you'd actually get, like, de-platformed if you criticized him before, it's rather funny.  I think he took a year off, right, before going to college, and so he's older.  I think for some reason he turned 19 when he was still in high school or something, which is--I don't know what's going on.

The line about David Hogg being 20 years old caught my attention, so I took three seconds to Google "David Hogg" + "age" and this was the first thing I saw:

So David Hogg is 18 years old, and he was the normal age for a senior in high school.  He won't be 19 for another three months.  Which begs the question:  Why did Styxhexenhammer666 think the guy was 20?

Beneath his YouTube video, Styxhexenhammer666 linked an article from The Hill about David Hogg getting accepted to Harvard, and that article contains this paragraph:

After the shooting, the 18-year-old and his fellow classmates reignited the debate around gun control, launching a youth movement surrounding the issue. The group helped to organize the “March for Our Lives” rally in Washington, D.C., that attracted thousands of anti-gun violence protests earlier this year.

The article is kind of vague about when Hogg turned 18 years old; I could see how somebody might read it and assume Hogg was 18 at the time of the shooting.  But even so, that still wouldn't make him 20 years old today.