There are other factors that drive employment of young workforce, which don't have much to do with knowledge or skills. These are sort of blue-collar jobs within IT.

There is a lot of low-level drudgery work which is currently outsourced or assigned to non-employees (mostly young) in most companies. For example, the IT support and maintenance is mostly done outside of the West. This works requires a lot of back and forth. Don't think AI taking over this area.

Also, some work is assigned to young workers to spread the accountability and risk ownership. Not sure if you can hold AI as accountable as humans.

Also, young workers are generally preferred for their agility, flexibility and ability to work hard. There were easier to exploit and if they were unmarried, they won't mind giving all of their time and attention to work for less pay. I used to work in teams that stayed overnight at office to complete projects. Young people are also very cohesive and they team up well.

So, looking at the situation purely from a knowledge perspective may not give the full picture.