Mr. 16
100' may be okay in your first year/week/day whatever... as mentioned it's not the height that counts, anything over 10 feet is dangerous.
I remember my "first", a norway maple, that to this day it isn't 40'... scared me you know what, at about 20'... stupidity, a little coaxing from a colleague, and I managed to live through the first few years of 'on the job training'... then I went BACK TO SCHOOL (post secondary) spent more time in the trees, learned the SAFE way to climb, and the faster ways to climb. Today I won't expect a new climber to be proficent, really solid, until at least three seasons in the trees... and even then a climber will still have alot to learn on just the basics.
Don't rush, don't panic about being fast, if you've had some training from work... take it slow and only do what's comfortable... then if you really think you like this gig... GO BACK TO SCHOOL! your too young to waste the oppertunity to learn lots while people are expecting you to. If you wait till later, it will only get harder to get back in the game. Look for Horticultural or arboricultural programs in you area, even if they are only 1-2 year community college courses, at least it will give you a grounding and a resume, other then "I'm still alive at 20"
My 2 canadian cents (almost 2 US cents these days!)