I'm not saying 25 is being old. Seen that Gladion is there too, I don't think they're applying the "you have to be 11", but still around 25 is a lot older than the other characters. Sun and Hau are 11, I think Lillie is a year older maybe? It is never stated but for me she was a little older than them. Gladion should be like 14 or so? And the grunt is below 20 for sure. So Kukui being among them is a little strange. Since he's with his Masked Royal persona maybe he's just a special guest?
And yeah, I can see the situation with Lillie being a misunderstanding. I doubt she'll battle in the festival
Well, the summary says the idea is for someone to take the trials to calm the tapu. So they could be ignoring it for the sake of allowing more people to participate.