I disagree. I was 22 when I quit my last job as a wage-slave, and that year I billed out 1100 hours at $125/hour. That might have been the best year of my life. A 6-figure salary for working essentially half-time.
In your case, I'd recommend trying to work 6 month contracts and always try to have a new one in the queue for more money, if possible, so you can ask for an increase at the end, or take the other job. But be classy about it.
I would take it one step further. Always have at least two active contracts at any given time. Real leverage.