43y here and definitely not old! I don't feel middle age either - reckon that's just a state of mind.
Mind you the knees went ages ago before my 40s due to arthritis (a firm not linked to old age) so I probably felt older back then when diagnosed compared to now when I'm used to it. And eye sight is better than ever due to laser surgery.
Also have more money available now so can go out and do more - so feel more active and out and about, so less old.
And dd is a teen so I'm not shattered firm lack of sleep - so again feel younger for that too.
Age is a number. 42y is not old these days at all, it's barely middle of your life expectancy.