Well I came into plumbing at 30. Paid for a course, did level two and three. Been going about three years now. Trust me it is hard work. Had to work for free for a while to get experience. then had to work for awful money for a while. You can still make good money if you can find your own customers and keep them happy, the big problem is getting enough of them. That takes years. The better money is in gas but getting gas experience is a nightmare. I've still only got about half my required hours after all this time.
I'm contracting on site now just because I need a regular wage to pay off everything I spent. Really the whole thing has been a crap load of stress.
But thing is I quite enjoy it. And if I'm honest I don't regret starting it despite all the hassle and money worries. I suppose as someone said above it depends what kind of person you are. I find plumbing satisfying work, and I basically can just get on and do my job which I enjoy. In the end I reckon the money will get better, but then I'm an optimist. I do know some plumbers round here are still making the cash, so in my head it can be done.
My advice is to try and find someone to work with as a mate/ labourer BEFORE you shell out for a course. It will give you an idea of whether it's right for you. And if you can't find anyone think about this, you are definitely going to need to work with someone after the course because you just wont be ready to go on your own. So if you can't find anyone after you have spent the cash, how much worse will you feel then?