All depends what sort of mechanical work you're thinking of.

General mechanical work on cars vans and trucks requires you to be supple because increasingly there's no room to work on cars, you have to fold yourself into all sorts of shapes to get at things and your arms need to be like those of Mr Tickle, unless you are the india rubber man you're heading to a time in your life when you start to stiffen up and all those injuries and strains over your life start to bite.

Simple repeat servicing is easy enough, but there will always be major jobs requiring gearboxes engines etc to come out.

Truck work is very heavy and filthy, a young mans game, those older fellows still doing it you seldom see smile.

Electronics or other specialising is what i would do if you want to get into this line, if you could sit at a bench of diag equipment and interrogate/ repair ECU's, fix faddish allegedly sealed for life LED lights that cost stupid amounts of money to replace, be able to overhaul and refurbish battery packs and the necessary equipment that makes them work, then that's something you could do for years and i would suggest make a good living from.

Not wishing to put you off here, i worked on cars for years as a kerbside cowboy, and still service and repair my own, i also have driven artics since my 20's, i like others started to notice various body parts seizing up and all the typical repetitive strains manifested themselves increasingly from about your age, once into my 60's car work proved inceasingly hard on the body, i'm still fixing and maintaining our own cars in my late 60's but a couple of hours now humping wheels etc about is enough, the problem in this country is the fetish for road salting, it not only makes everything underneath filthy it corrodes and seizes things up so what should be straighforward dismantling after a few years of salting becomes hard work.