Are you going to ATP? I am a flight instructor there if you are. From your 90 day talk, it sounded like you are. Your written expires 2 years after you take it, unless you continuously work for a 121 carrier and upgrade at a 121carrier. If you take your ATPw, and don't work at a 135 or 121 carrier, i.e. you flight instruct for 2 years, it will expire. I wouldn't take the ATP just because I was 21, I would wait until you are getting ready to apply to an airline.