thinking about how viggo is such a good narrative foil / parallel for hiccup, and one of the things that popped into my mind is that it's actually really important to their dynamic that viggo is significantly older than him, and not someone around his age. (and this is a pattern that we see with a lot of the franchise's antagonists, but it particularly stands out to me with viggo).
because viggo is, essentially, the type of person hiccup could have become if he hunted dragons instead of trained them. and he has so much more experience hunting dragons than hiccup does training them.
and hiccup's story has always been about overcoming these impossible odds, about the weakling, the underdog, standing up for himself and accomplishing great things. and that's exactly what happens with his conflict with viggo. he goes up against this person who represents what he could have become, except viggo is stronger, more experienced -- and hiccup still wins anyway.