Hello, entering the fray with some trepidation........
I've known a few people still doing yard work at your sort of age and really enjoying it but not that many, I guess it depends on the individual!!!
I would disagree with those who say better to groom than instruct, I'd go vice versa. Someone mature, clued up and with a good sense of safety and also empathy will always be valued as an instructor and those with the patience and knowledge to teach young children and nervous adults and bring them on sympathetically are not generally that thick on the ground. The money is better for teaching, full stop! I've freelanced as a groom/ rider/ instructor and the scale on which I charged for each thing was bigger from sh** shovelling up!! If you think you might enjoy it I would suggest that you take the time to watch lessons with someone you respect and see if it lights your fire, so to speak!
I think what has been said about the AI is a bit harsh, although I do believe that the qualification is given too much weight by many. However, I've got it and it was a really useful experience to go through, I've also got my stage 4 but neglected to put in for the exam for the teaching in the same run of exams which was stupid but the moment passed and I never got round to doing it, I wish I had. I have found those bits of paper very useful and the whole experience of doing the exams really made me think. So I would say all to the good. I would second the opinion of those who have said that equine degrees frequently (but not always) churn out desperately impractical graduates who fondly imagine that they will make thousands in a marvellously paid job......
Equine dentistry can be very physically demanding but is another option as someone suggested.
Finally I'm 38 and no longer work with horses. I still teach flatwork to friends if they ask but only if I trust them implicitly and no longer teach jumping as I am no longer insured. I still miss some aspects of it and look back on the many memories with such fondness that I consider it all over again and then remember the following which might be worth double checking you've considered....
Muddy fields on winter days are low on fun factor, particulalry when they involve changing rugs on unimpressed eventers.......
Everyone wants lessons/ grooms in the evenings and weekends......bit like I would, which currently would not work for me or anyone around me!
I bounce less easily than I did
Other people's nutters seem to be getting nuttier than I remember!!!
I'm pretty laid back and tend to think "oh well if boss lady wants it like that fair enough" but I've worked with a lot of people who were driven bonkers as they got older by seemingly inanane requests!!!
Sorry for the ramble, good luck whatever you decide 
I've had some fab horsey jobs and to be honest if you get some experience behind you and look around and could offer some child care skills as well you might find a little gem out there for you! Despite the above I'd probably consider it again if my circumstances were different!