There is a much simpler explanation: growth. Most of the companies you're thinking of when you imagine software companies are very new -- much newer than "old"

There is a much simpler explanation: growth. Most of the companies you're thinking of when you imagine software companies are very new -- much newer than "old" engineers. Meanwhile, many of the best engineers are only ever on the market for a new job once, when they go looking for their first job. The number of developers grows every year as well, with far more new entrants coming from younger generations than older ones. As a result, most of the new developers you see will be younger. As long as the market needs more qualified developers, the growth will favor the young.