I've just completed this course this year. I did the physics pathway though so didn't do biology. There are some mature students on the course but it depends what you mean by "mature". Most are late teens and early 20s. There was one guy in his 30s and two in their 50s. (I was one of the latter two).
It's a good mix of people and as mentioned it's run by the LLC (Lifelong Learning Centre). A bunch of us tended to use the centre as our rest place between lectures and for lunch. They give a lot of help too with office hours for when you're stuck on anything. Tutors are really good too and are really very helpful and enthusiastic.
I went into this course having only re-studied 4 GCSEs but not studied any A-level content. I did OK averaging 76% across the board. Some students did extremely well getting into the 90s but there were a few dropouts along the way too. It is an intensive course but generally starts off with higher end GCSE stuff. I'm really glad I redid those GCSEs as they stood me in good stead. Lack of previous A-level study for me didn't seem to matter much, but if you've done any already (as had many of the cohort) you'll be fine.
There is a Skills module you have to do as well. This is where you write reflective essays early on (short 300 words) and it can get a bad press from students. In semester 2 this grows into a student project and you need to do a report and presentation. But they do teach you how to use the library function on the computers and how to reference properly for academic essays. I'm going into year 1 engineering at Leeds now and I feel already at home there. No worries about what the place is gonna be like. If you pass you're guaranteed a place on your chosen course. But they do limit the number of people who choose medicine and mech.eng.
You're not stuck on the choice you make when you apply either. I chose mech.eng on my application but ended up selecting mechatronics and robotics. You just can't switch pathways between biology and physics degrees obviously as you take one or the other. Chemistry, maths and skills are core subjects for everyone.