I think some, perhaps most of this effect is related to the growth of the industry. Far fewer 25 year olds started a career in coding in 1990 than 2015. Fewer 25 year old coders in 1990, fewer 50 year olds in 2015.

That dynamic has existed for a while. When Gates' & Wozniak's generation started coding it was an unusual pursuit. Age gaps became part of the demographics. Like everything else, being common translates into being normal.

Other than that, I think there is a little more attrition in coding. Experience is important, but current knowledge is too. SV startups are riskier, so more appealing more to younger people. They use new technology, so less of a comparative advantage for old skills.

I'm not saying discrimination doesn't exist, I'm just saying that demographics play a big role explaining the average age. Some of the discrimination that does exist likely comes from simple human pattern matching. That's not what a coder looks like. All the coders I know are 25-40.

If I'm right, time will fix this.