Good on ya!
I was 28 when I made the change, previously working in IT / marketing / sales (ewwwww).
To see whether I liked the outdoors, I tried some volunteer weekends with BTCV in Kirkstall, Leeds, then signed up for a BTEC higher national diploma (then Houghall College, also maybe look at Askam Bryant to see what they offer), got part time work with one of my fellow students who was seasoned and never looked back.
If you don't want to start with the academic side, at a minimum you'd be best getting CS30 / 31, maybe even 38 (they're all called different way complicated numbers now, but chainsaw maintenance / cross cutting, small tree felling and climbing basically). If you fancy a go, maybe even CS39 (climbing with a saw) but if you get in a with good firm, starting on the ground and learning from your team before progressing into the trees can be handy too.
You could also look at doing one of the longer crafts courses where you do so many weeks residential with practical tickets and some theory (Merrist Wood, Kingswood Training, Plumpton college, etc.).
Alternatively, you could look to do some practical tickets on your own and some part time theory on the side (maybe the ABC level 2 certificate in arboriculture - some offer this at college, others distance learning - Tree Life, Myerscough, etc.).
There are also other higher level part time courses (such as the ABC level 4 diploma) but generally you should have some experience before trying this (The Training Tree, Merrist Wood, Tree Life, etc.)
Sorry if I've missed training providers! Try this link for the Arboricultural Association guide to careers:
http://www.trees.org.uk/Trees.org.uk/files/5d/5d1f7bb0-68d0-4164-a65c-89e026ec71d7.pdf
And their guide to training providers:-
I hope this helps and feel free to PM me if you want more advice or information.
If you're looking for work, there's a range of places to try
http://www.ctcrecruitment.co.uk/
https://www.treebear.co.uk/
https://arbjobs.com/
Good luck.
Arboricultural Association - Directory of UK Arboricultural education and training providers