If we look past the numbers and mathematical term for middle, the stages of life could be determined by how capable or productive a person is. In that way, 50 is still a bit high, but it's close to the peak of productivity almost regardless of the job at hand.

On a positive note, we should be happy if this "middle-age" increases, because it means that we're more healthy and capable for longer. This is also very visible. Just during my life (mid 40s) I can see that the people of today in their late 60s look and behave as the people in their 50s did in my youth. It's like the capable years have been extended by 10-15 years.

On a more depressing note, the expected lifespan hasn't increased that much in the meantime, so it's not exactly linear. It seems that the change from being capable to being incapable due to age is really sharp. People don't enjoy long retirements the same way as before.

You know how working 5 days a week to have a 2 days off is bullshit. You can never do all the things in the weekend that you dream about all week. Same thing about retirement. You'll never get to enjoy the carrot at the end of the stick.

If you want to do something, do it now. If you can't do it now because of obligations, you need to change your obligations. Seize the day.