My Mum was old at 45 and has been so for 20 years now. My Dad is not! I would call him mature, yes. Adult, yes. But never in a million years an old man. He is enthusiastic, open to new things, has a keen sense of humour and physically active and very fit.
Whereas my Mum just loved the idea of being old and jumped in at the first opportunity, i.e not interested in looking good or feeling good, trying news ways of doing things, getting engaged and involved with the modern day. She is happiest in her comfort zone, in the house, doing gardening and watching telly. While Dad tries to get out at every opportunity. The numerical age difference between my parents is 6 months.
My husband is 43, but no way is he middle aged. Does not look it or act it.
However, I felt overwhelmingly middle aged at 34 two years ago, completely worn-out by children and pregnancies, with my fat and flabby figure and dwindling energy. Felt I had more affinity with 50- and 60- year olds than 20-year olds. I absolutely hated the feeling of old and past it. But two years on, kids are a bit older, there's more time for myself. Started working out for the first time in 10 years and with every month I am feeling younger and younger!!