Yeah, 40 is the traditional 'over the hill' birthday.

My definition for middle aged will be 40 until I reach 40 at which point I will shift this definition to 45.

"Most people" live until their late eighties (at least in Western Europe), modal and median ages at death are generally 86-90, whereas the mean is lower as you have around 15 years to die in after 85, but 85 years to die in before which pulls it down