My grandson and I have thoroughly enjoyed this series as well as the two spinoffs, 3 Below and Wizards of Arcadia.
The storylines are well written with the perfect balance of action, comedy, and drama.
We love the way characters have migrated from one series to the next adding to the continuity of the storyline from the original Trollhunters yet each series having a completely new storyline as well.
As for the character design and animation, masterfully done! And I would have to say the best I have ever seen from Dreamworks along with the How To Train Your Dragon franchise.
If I had to guess a good age group for these three series, I would have to go with 12-15 and possibly 10-15 depending on the ability of the child to follow a slightly complex storyline and their love for science fiction and fantasy.