I agree that the "young adult" franchises have been done to death. I think it would be more interesting to see how adults deal with the sudden manifestation of powers in their lives rathe than the usual "teen angst" and drama from a teenager discovering they're "different".
If they did decide to have a teenage character, it should be a character that the audience can see grow over time into an adult within the MCU (so at least 10 years played by the same actor).
As for whether the youngest character should be male or female, I'd say go male for a change because young females are often used as morality chains for older (particularly male) characters with an older female character usually "taking them under her wing" then dying so that the young female can learn and grow and "step up". With a young male character, they can either be led astray by an older male character who they're trying to emulate (despite being warned against it by another team member) then it backfires and they learn their lesson or they can try and bond with one of the older males but be rejected because the older males "don't want to hang around with a kid" and eventually they step up and earn "respect" for being a valuable member of the team.
Alternatively, they could go for a set of twin characters (1 male, 1 female) and show their different experiences within the group.