Hi Rob,
I've been 21 now for about 10 days, and am three exams away from completing my ATPL exams. I'm pretty worried that on completing my course and sending out countless CVs, I will hear nothing from the airlines, etc, partly due to my age. I had the same thought about instructing, I could imagine the whole 'How old are you?!' type thing. I'm considering working abroad for a year or so, if I have to. Whatever I have to do to get paid to fly.
I would say go for it! Good luck