You can’t fly long haul (787, 744, A380, A330) beyond the age of 65 due to overseas licensing requirements. US, Indonesia and a few others don’t allow a pilot of an airliner to be 65.
Most people hitting that age give it away. Some don’t want to or have to keep going so they bid to the 737. It’s a very small minority of maybe 3-5 per annum.