another thing to note we dont even know what characters and what ages some of them may be. They could always go for example flash a yr or two older but is held back due to bad grade, and mj if she isnt there from the start and comes into midtown at a later point she could be a little older. But personally going from the 17-early 20s target we are likely to get as long as they dont really look so old. I dont see the issue if say mj or gwen is played by a 22 yr old playing a 15 to 16 yr old. Again we all had this talk throughout the board. There is plently of examples in your old life and with actors who are much older and look 5 to 10 yrs younger and the reverse younger aged but look older.
Heck i see it in real life all the time. With the kids today and they way they wear cloths and makeup there is folks like 12-14 who look like they are 18-22 yrs old and you wouldnt think that and so forth. So again as long as they are not looking like they are late 20s/30s, greying hair, or wrinkles in the face the actual age of the actor isnt the issue.