You can always change your target audience. If you want to appeal to the teenage crowd, gear your content towards it. You may have to research current trends as you might have been disconnected from the teenage reality for too long. I'm a late bloomer when it comes to a lot of things and I have regrets not starting my channel sooner but, 20 is good age, trust me.
Before the information age, record companies would encourage musicians to age themselves down to appeal to teenagers, even if the group was in their late 20's.
Once you enter your 20's social age gaps tend to get a lot wider. I'm in my mid 20's myself. Some of my friends I connect with are almost 10 years older than me but, I can still connect with college freshman as if I was 19. No one really sees anyone as being out of place.