I personally don't think FF works as teens {at least not the whole team} them being different age's/at different points in there lifes, is what rounds out the group dynamic
making them all the same age or all in school together/ at same time, kinda takes away from them each bringing they own unique prospective to the team
I mean, you could make Ben and Reed closer in age (since they have a backstory of going to collage together) but even then Reed is suppose to be some genius prodigy who finished school at like 15-16... so, Ben should be at least a few years older (having had gone to collage at a normal age)
Sue and Reed could be a similar age without disrupting their dynamic too much (if say, he met her while teaching collage classes when she was a student) having had already finished school and began teaching by the same age she was still in school (tho, that makes for a questionable dynamic, when it comes to them getting romantically involved)
although, it still gives them 2 very different life experience {even a the same age} to make them unique
but, Johnny should always be the youngest (and him and Sue should have a decent age gap) to differentiate where their at in life... and give her that older sister dynamic