A Movie About Perfect Sacrificial Love
For our family, which has grown up with this best of all franchises, this was pretty much the perfect end to the perfect story arc. That said, it will mean a whole lot less if you haven't seen at least the first two movies, and I strongly recommend also watching the excellent animated series (all 8 seasons' worth). My age rating is based on "will get something out of" but there's nothing offensive here at all, my only caution being that some of the antagonist dragons might scare the little ones. I disagree that the romantic angle is overplayed. It's such a trope that the big, macho scary male falls apart when confronted with girls, but this is the best I've ever seen it done. Smart, fast, fierce Toothless (is there anyone who doesn't love this character?), whom we've always been led to believe is the last of his dragon species, comes across a female of his kind. Hiccup, the human main character, attempts to coach him through a courtship dance, which is utterly hilarious because it's the blind leading the blind (or maybe the Nerd leading the Clueless). Toothless coming up with an original approach is a wonderful shout-out to the most iconic moment of the first movie.
Ultimately, though the movie is funny, thrilling and a host of other good things, the plot is about perfect sacrificial love. Parents may get this more than children will. Kids who follow the series will be sad at the ending, but I think it takes an adult to grasp that the supreme act of bravery and selfless love is not keeping those we love close but letting them go when that is what is best for them.
One nice extra for the whole franchise is that it portrays young love (also love between adults) as something whose best manifestation arises from firm friendship, slowly rather than at breakneck speed. If you have kids approaching or at dating age, this is a good talking point for this franchise, as it gives an excellent example of this in "Hiccstrid." It may be a little corny, but the relationships as portrayed are blessedly free of R-rated material, and that is such a rarity anymore that I could have given the movie lots of stars for that example alone.
Bring lots of tissues; there will be tears!