I think it all depends on Disney/Marvel's long term plan. If Disney/Marvel truly want a teenager to grow into the part then O'Brien and Lerman are too old. Plain and Simple. If its a teen they're casting, their out, just as much as Bryan Crantson was out as Lex because Snyder clearly wanted to cast a younger actor like Adam Driver, who was apparently up for the part, and Eisenberg who got the part. Its that simple.
Yeah, O'Brien and Lerman could conceivably play teenagers at their age now, but if they truly want a young actor in the role , there are all kinds of reasons you could put forward to cast someone in their 20s, but in the end, the production has already made its decision.
It then becomes a question of whether the production has made the right choice in going with a younger actor. In the end , I gotta defer to whatever choice Disney/Marvel/Sony have made in terms of the actor they want to go with because they know the plan, they know what they're looking for, and they have the script. These guys do this stuff for a living, and at the end of the day, we're just fans giving our preferences.
They have to go with their vision of what Spiderman is gonna be , and its either gonna work or it won't.