Middle aged is how others see you.
I’m 54 yet I still have the same 32″ waist, I have a resting HR of 48, can road cycle 100 miles, ride my MTB faster down a hill than a bloke on a full DH rig, ski any black or skin up any mountain and apart from a couple of aches and sprains have no illness that I’m aware of. I think of myself as about 25 mentally as I think I’m open, interested and keen to learn about new things. I see blokes my age and can’t believe how old they look and behave, it’s as if most people aged 54 have accepted that they will slow down and get ill.
Yet I’ve seen a bit of life so inevitably I have some ideas on how I think things should be done; when I express these ideas people (my family) deride me as old-fashioned and fuddy-duddy. But I simply don’t see myself as middle aged, for me that’s the ultimate term of derision.