I feel like rick missed an opportunity when he made Frank closer to Percy's age instead of Hazel's. Like, I'm sure there are reasons (It's been a while since I read HoO, so I can't say what they are, but they're probably there(I hope)), but can you imagine Percy, on a quest with two kids that are 13-14 (I can't remember how old she is in SoN, can't find it either, so guessing low it is).
Even without his memories, he'd be halfway to feral, but once he got them back, I can't see him doing anything but going from protective to absolutely destroying anything that hurt them.
I also think it would have been hilarious to have Percy and Annabeth at like, 17-ish? And have the rest of the seven at like, 14 max. Percy and Annabeth are trying to keep this group of 13-14 year olds alive, and they're both screaming internally half the time. Sometimes, it's just screaming, but sometimes, they scream about how toddlers shouldn't be on quests.
(Side note this popped into my head while I was writing, so have a- thing
Annabeth: There's a bunch of babies, Loose on a quest! No one knows what the babies are going to do next, least of all the babies! They've never been on a quest before!
Narrator: The babies in question are 14, and they have, in fact, been on quests before. (She is right about them not knowing what they're going to do, though.))
Then again, maybe I just like big brother/protector Percy too much. (Reluctant and screaming big sister/mentor figure Annabeth is also fun.)
(And on a sadder note, Annabeth realizes this must be what Luke and Thalia felt like and almost starts crying.)
To start, you're absolutely right that it was a missed opportunity. Percy is, without knowing it, the most experienced of the questers, except Frank is nearly his age in comparison to Hazel (who should be 14 by math, but is 13 by Rick's declaration). I think it's less that he made Frank closer to Percy's age and more that he was trying to make Hazel younger than Nico.
But yes, it would've been super interesting to see him basically mentoring the other two, especially after he regained his memories. I feel it was a major missed opportunity to not have Percy and Annabeth trying to watch over the younger quest members, instead he aimed for weakening the two to make everyone else their equal. It's a shame cause it missed the opportunity for the younger questers to be major boons for their skills, rather than raw power.
And also, the two of them looking after younger ones and wanting to protect them but also having to risk their lives to save the world- and the rest being alone when Percabeth falls with Nico to help except half of them find him freaky- would've allowed them to grow into their own more.
It would also show how Percy and Annabeth really ARE the most experienced, how their quests haven't been the norm so even Jason who is remembering some quests hasn't done anything this insane before but Percabeth just takes it all in stride.