I'm a self-employed plumber/pipe fitter,
I started the week after I left school at 16, qualified at 19 and finished my apprenticeship at 21, if you do look into getting a trade then try and get and evening course or part-time until you get your ticket, don't do one of those 6 weeks course and here's your qualification because they don't work, you will be on a job without a clue what your doing.
If I could give some advice it would be learn to be a sparky before a plumber. It's cleaner and IMO better for roughly the same money.
It all depends on what you want to do, if you want to do maintenance work in occupied property you will have to do call outs but the work is easier, if you want to get into installations and building sites etc there is no security in your job.