Yes, you can become a plumber at 30. You will probably need to do night school to get the qualification, and will need to work as a plumber's mate (dogsbody) at the very least just so you have some sort of hands-on experience and understand the trade, otherwise you'll struggle to understand the job.

If I were you I'd have a look at the people you already know, extended family, parent's mates, friends of friends etc, and see if you have anyone who is working in a skilled trade, electrician, plumber, tiler etc.. then ask them if they know anyone who needs a hand, then you have an in.. you won't qualify for an apprenticeship at 30 afaik, so just find a small firm that needs a helper, work your way up, do night school to get qualified.. it'll be tough, you won't earn much and you will be burning the candle both ends, but in a few years, maybe by the time you're 35, you'll be set to start off on your own and make serious money, then by the time you're 45 you might have employees, and 45 is still young these days so don't think you left it too late, you haven't.

don't limit yourself to just being a plumber, have a look around at who you know and what they do, if you know a good electrician and they're willing to take you one then that's a huge benefit that I wouldn't turn down.