The OU don’t do a teaching route in itself. They do childhood studies but doesn’t qualify you as a teacher.
They do however do biology and plenty of other subject degrees if you wanted to teach secondary.
You would then have to do your one years post grad teaching after that.
As for studying and working full time-many of us do it. And yes it’s hard, yes it can be time consuming but it’s not impossible. It all comes down to time management. You can also mix it up so you start full time and then split your final year into two so it would take 4years. (Full-time would be 3years, part time would be 6years in general)
Funding depends on where you live but your age doesn’t factor in.