Yep, the whole age thing also greatly bothers me. The 'old' characters acting like they know so much about everything being 20-22. I would expect Lulu to be in her late 20s or begin 30s. Yuna being around 23-25 would be realistic. Rikku can be debated more. There are teenagers who look older...I would say between 17-22.
This also really surprised me the first time. It was a good twist. You also brought up another good point. In that character interaction being really decent, even with the limited dialogue or cutscenes. It is easier to relate to characters who prefer interacting to specific characters or genders. By that I mean Faris having many alone time with Lenna. Trying to relate to each other, being curious about each other and become closer. Finding out that they are sisters was not ignored. JRPGs have the tendency to suddenly have party members with family or other connections while ignoring this part entirely. Or Bartz somehow leaning towards Galuf due to being fatherless. This last example might be my own interpretation but it feels like it makes sense, when they seem to have more men-talk or speak with each other more.
I'm in the second world. Xezat died inside a tower and going to Ghido's Cave with a submarine. That's around 60-65% of the game.