This is an observation that in general technology worker pay tends to plateau at about 5 years worth of experience. It is not that it doesn't go higher, but the rate of increase significantly drops at about the 5 year level. That is unless one goes into management or quasi-management (which is not "tech pay" anymore).
("Plateau" may be the wrong word, but it is hard to breifly describe a curve with a kink at 5.)
It could be SoftwareAgeism, or it could just be that experience itself is not heavily valued, regardless of the worker's age. I suspect it is a combination of: