Your chart seems to indicate software development related intelligence peaks somewhere between 40-60. Given this, why do software companies like young people so much?

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.

The chart of intelligence vs age is terrifying in a sense, I'm amazed it's so clear. When I was in my 20s working on my phd, it's true I was learning a lot more new things. If I pursue a new phd at age 60, I wonder if it would push my ability up. Nothing I use in my day to day job was hardly invented 30 years ago, I've had to learn everything. I think most people not in fast changing engineering fields don't get that constant mental stimulation.

I did a masters and then a juris doctorate in my 20s. I am now in my early 40s, and I am doing another masters with the goal of doing a PhD when I am done.

Before I started, I was pretty concerned about whether I would be able to do this even. Most of my peers are in their 20s and 30s, though it was surprising to me when I started this program that I am not the oldest candidate and there are a couple of us in the higher age ranges. Anyway, I had wondered if I would be able to keep up with the youngsters. In fact, I don't think it has been an issue at all. I think that I am performing at least at as high of or possibly a higher level today than during my first run at graduate education.

I think there are certain factors that I, as an older, more settled, less let's-start-living-life(!) person have as an advantage over my young peers. Many of them still focus a lot of time on socializing, whereas I have my spouse and kids and old friends whom I don't have to see every weekend (the old friends--I see my spouse and kids every day). My younger colleagues do a lot of dumb things still, ones that I sure did also when I was that age and have gotten out of my system.

I would say that there were certain advantages to doing graduate work when I was young, and certain advantages now. I find it impossible to say which is better, but it is at least no worse now than then.

Age causes brain volume decline and intelligence loss, like distance causes tiredness in marathon runners. i.e. it can't cause anything, it's an abstract measurement - age doesn't cause rust, oxidation does. Distance doesn't cause tiredness, running does.

Increasing age and increasing decline can't mean that age causes decline, it shows one or more of: processes which cause decline keep happening, repair processes either don't happen or get worse, new generation either doesn't happen or gets worse.

(If worms were eating your brain, your brain volume would go down and your intelligence would go down as time passed and your age increased, it wouldn't prove that age caused your intelligence loss. Correlation isn't causation).