You’re middle aged when your growth hormones still exist, but the levels are starting to drop. You know you’re middle-aged when you twist your ankle and it doesn’t heal for a really long time.

When you start getting the injuries that don’t heal, that’s when you know.

It’s not a numerical thing. It’s a phase of human life. As we extend lifespan with new medical technologies, the stages won’t stretch out to fill the lifespan; we’ll add new phases at the end.

Those phases will be very strange, because they will involve drastic steps to maintain cognitive function in an aging brain. It could involve increasing top-down control of biology, or it would involve replacement of biology with machinery. Either way the new later stages of life will be stranger and stranger, and will feel different than any human experience does now.