Something we all need to consider:
There is no way Spidey will be in high school for a considerable amount of time. Even if they cast a 15 year old/made the character 15, he'd be 20 by time the second Spidey solo released in 2020. If they cast a 20 year old, he'd be 25.
So age doesn't matter. Because the character will be out of high school by time they reach the second film.
Exactly!
PP age in different films:
Civil War: 15-16 (Filming 2015)
Spider-Man: 16-17 (2016-17)
Infinity Wars 1: 17-18 (2016)
IW 2: 18-19 (2016-17)
So by IW he will already be out of HS. (That's also assuming they don't pull a "he skipped a grade since he's so smart" which I wouldn't mind.)
Guess what though, IW 1 and 2 will start filming NEXT YEAR!
That means if they chose someon who is 23, for instance, the person would be 24 by the time they act as a College aged Peter. For all we know there will be only one spidey solo in phase 3. However, I would assume that there would be at least one more. There are rumors that they may plan on filming 2 and 3 together. In that case they can film:
Spider-Man 2: Age 17-18 (2017)
Spider-Man 3: Age 19-20 (2017-18)
When the actor would still be 25.
Even in that circumstance though, both sequels could be in college for all we know. Otherwise at least the second one would be.
If ANYTHING we NEED someone whos age look does not change too much during the filming, because if they do more than one solo the age of the actor would be older when PP is younger. (Eg actor is 24 filming IW1, but 25 or 26 when filming Spider-Man 2 which would be released the same year.)
So who can maintain a similar age look for a couple years? Someone going into second puberty, or someone who has past that but is young enough to not have their age look change too much?