Because a 16 year old playing a teenager means 10 years later when they make the 3rd movie, the guy will be a 26 year old playing a 20-something guy, vs a 36-year old or whatever doing the same. And 15 years later the actor still wouldn't be too old. It's for longevity's sake I'd imagine.
Ten years? They've had three Iron Man films between 2008 and 2013. If Spider-Man is as popular as I'm sure they hope it is, they're not likely to drag out the releases by that much. I was thinking that they'd more likely go with about two years between the solo films - so 2017, 2019, and 2021, plus appearances in other films.