Nearer 50 than 40, but I’ve been very lucky and don’t feel to have any real problems. Back is a bit stiffer than it used to be, but that was due to a slipped disk 15 years ago which has not troubled me since. About as fit as I’ve ever been, running marathons (which I never did when young) and although I’m no great talent, I’m towards the quick end of the local running club. Hold my own in cycling events too. All my own teeth (no fillings), no specs, bit thin on top but who cares. Haven’t noticed the grey matter failing, but maybe you don’t until it’s really bad??
I blame cycling to work every day. Though of course I’ve been lucky too.