It depends what you're doing now and how mature you actually are.... if you're nearing retirement age then maybe it's not a great idea to do a PhD for career reasons, though we do have two retired students doing theirs for pure pleasure (presumably). If you're using that 'official' definition of mature - what is it, 25 and over or something? then it depends on how long you are thinking of working for - will you be happy doing an unfulfilling job for the next 30 years or so, or do you want to give something else a go?
How different is what you're currently doing now to what you want to do? If being a lecturer is your dream, where do you want to be doing it? You don't have to do a PhD and work in higher education, there are other parts of it you could teach in - FE, adult education, school age kids, the private sector - it depends on what your subject is and what level you want to be teaching at.
Yes, the job market is competitive, but doing a PhD helps you get into it - you become an 'expert' in your field. You are likely to get teaching experience related to your subject, so that's a toe in that door. All the jobs I would like expect applicants to have PhDs and a good research profile, as well as the ability to teach, so not doing one wouldn't be an option these days. I already work in academia, but I haven't always - had a gap for a while doing something completely different to my academic background, so doing my PhD helped me get back into it. I know what I ought to do after completion to end up with a job I would like and I know it will be a lot of work, but there's no point in not trying. Anyway, sometimes opportunities pop up along the way and it turns out fine, but in a way you didn't envisage. I'd hate to be a little old lady full of regrets about what I didn't do in my life, when it's all too late to try, but it's different for everyone I guess.