I went an did an access course at 21, not really having a plan but wanting a challenge and some direction. Then I went back at 31 to do a biology degree, again with no real plan, but I'd been really interested in it at alevel, and being tired of sales jobs. That was the best 3 years of my life and I'd love to do it all over again even if I wasn't getting a qualification at the end of it. Can't do much with a biology degree, and I really enjoy studying so I'm about to start a masters degree (found a passion for conservation whilst doing my undergrad), and hope to do a phd after that.
I have two kids, one grown up, one almost grown up, so now is a good time for me to begin a career.
As for why uni as a mature student is a good idea - there's loads of mature students at uni. By the time most people graduate they would be classed as mature students too so you're not out of place at all. Most of my uni friends were under 21 and we got on great, age never came into it. You also have the extra wisdom, life experience and (probably) dedication as an advantage All those time management and organisational skills from having kids or working for years really, really help when it comes to deadlines and things.