Not really. I mean it stops an airsoft shot, but it will not properly defend you during a terrorist attack, or nuclear fallout You can buy airsoft plate carriers, which could technically hold bullet proof plates in them, but they are not meant for this and should not be used in that way. But you can buy airsoft proof plates which will absorb pretty much the whole force of a bb's impact, effectively protecting you from airsoft guns if that is what you meant.
It depends. If its a league of airsoft teams i would say 18+ but you can always set up your own games with other people.
by using science and the materials
An airsoft pellet is capable of damaging walls. In most cases, it would be more likely to dent or cause chipping, but all building materials and buildings are not created equal.
You should build it out of lava proof materials such as poo
transparent Airsoft Guns don't really have a name. But people would refer to them as "Wal-Mart guns" or "Cheap Chinese Crap" since they are so low-end and the materials are very cheap.
evike, airsoft megastore, airsoft gi
There are many materials with bullet proof properties including Kevlar, Lexan, Titanium, Steel and Carbon Composites. Keep in mind that "bullet-proof" is a relative term. Armor-piercing bullets are designed to be able to penetrate most of these so-called "bullet-proof" materials that will do a decent job of stopping normal (non-armor-piercing) rounds.