Based on the accepted answer to the question you refer to (Why does earthly punishment start at 13 and divine punishment at 20?), it seems that
teenagers really should be taking the Yamim Noraim seriously.
To be more specific, the accepted answer lists 4 caveats from the Chacham Tzvi (Response 49) about the under 20 rule.
Let's analyze each one:
- It was only before the giving of the Torah that punishment started at 20, after the giving of the Torah it starts at 13. (I believe this only answers why we say Sarah was free from sin at 100 as when she was 20. It does not address the Gemara or any others who say that Heavenly punishment starts at 20)
Since you're living after the giving of the Torah, you're now liable after 13, 7 years before 20.
- The Heavenly Court doesn't always punish for crimes committed under 20, and that's part of Yitzchok's "deal" with G-d, to ignore any sin committed before 20.
Doesn't always punish - but sometimes it does; why take a chance?
- The Heavenly Court doesn't punish people in this world for doing something wrong when they are under 20. Once they go up to heaven, however, they are judged for everything done after 13 years old.
If you - Gcd forbid - go to heaven before your first post-20th-birthday-Yom Kippur you're going to be in trouble if you didn't do Teshuva.
- The Heavenly Court waits until one is 20 to see if they do Teshuva. Once the person reaches 20 without doing Teshuva, the person is punished for the sin he committed when younger.
As in above, you don't want to risk being called in before you get a chance to do Teshuva. In addition, you will need to keep track - for 7 years - of everything that needs fixing, as you're not off the hook, it's just a deferment.
So it seems that even for teenagers the Yamim Noraim are not practice runs, but need to be taken seriously, since heaven is keeping track - as per each of the answers above.