To steal from patio11, an employment contract is both payment for current services and also an option on future services-- hiring people is hard, and you want to keep them around.[1] A 60 year old is less valuable since they're more likely to retire than a 30 year old.

1: https://news.ycombinator.com/item?id=21177759

Most places treat employees like crap, or structure compensation so that you need to bounce between places.

As to why old people, higher skill levels in many cases.

That would matter if people in software didn’t move so frequently to other jobs.

Sadly because hiring people before they wise up to the employee/employer relation is extremely beneficial to the latter.

Most software development isn’t high skill work.

More work, less pay.

People between 40-60 tend to have things they care about outside of work at higher rates.

pay them less, easier to get to do stupid things (work really hard). but its a stupid choice.