Hi mate, im 25 and been a plumber for 6 years now and I started off domestic working for a small business, then on my last year I did commercial mechanical, the reason u should start domestic is so u can learn all the basics of plumbing for a house, this will help u in the long run for cashies that never end, once your confident get onto commercial, that's where the big bucks is and good clean work !most companys would have want u top have completed a pre app!! after u do the pre app then I can give u some number!! hope this helps!!