I finished my nurse training last year, there were quite a few people on the same course aged 40+. Age isn’t a barrier but it’s getting tougher every year to successfully enrol for nursing due to the numbers applying. If you’re not already working in healthcare you’ll do well to start volunteering in the sector to gain experience, also if you’ve been out of formal education for a while you may be required to do an access course at college before applying for uni. The course itself isn’t overly intellectually challenging but will take up a large part of your life.
I’d advise to closely consider your reasons for wanting to enter nursing, qualified nursing is very focussed on an administrative role these days sadly. You don’t need any particular qualification to become a health care support worker and the job may be more satisfying depending on your motivation.
Good luck if you decide to go for it.