I am forty and still thinking of taking up Aikido. Thing is, some of those throws look a bit difficult. Has there ever been on record anyone who has rolled and broken their neck on a first uke?
I started Aikido at 40 in 1993. Admittedly, I had a judo background from 72-80, competitive rowing for a few years also. Started Aikido with a very tender lower back, was receiving relatively regular treatment for it. After a few months of Aikido the physio said that my back was stronger, more flexible, and more stable than she'd seen it.
I'm USUALLY ok... Little niggles tend to take longer to heal, but I'm usually fitter with Aikido than without. I've had long gaps with little training, and it has only been after a few weeks back at Aikido after a gap, that I'm fitter, more able to work, more able to move without a stiff hip and back...
If you're 40 and have a sports/fitness background it will take a while to get your Aikido up and running, so to speak, because in a lot of movements, it's a lot like learning to walk over again.
If you're 40 and have a no sports/fitness background, it will take a while, and partly it will take a while to get your fitness up, but as with most new activities, the fitness and the skill rise almost hand in hand...