Not much to discuss about, I think. This problem exists in most countries, and does not concern only pilots but more or less any kind of job:
Once you're beyond your late forties, you become unappealing: you may be experienced, but you MIGHT also have difficulties to adapt and change in your ways, you might also be a potential source of trouble, and you might also develop a health-condition which renders any investment in you worthless (as far as the company sees it).
So why take the risk, if there is somebody else available who's sixteen years younger than you.
A new employee may have that much less experience, but he will be very motivated and eager to learn. He probably has less expectations, because he hasn't seen anything better yet. He just needs some training and the prospect of making personal progress, and will be happy with even less salary.
So being more specific about pilots:
The older guy with 12.000 hours and experience on five different aircraft types in four airlines, who can fly the perfect NDB-DME approach with only one engine and restriced flight controls - he will expect a high salary, he will argue with the checker inside a simulator, and he will talk about all the benefits he used to have in his former airlines. He will also stand up to management with self-confidence.
The young guy, with only just 500 hours on a jet on the other hand: He'll be happy to accept a small salary, if in turn he can expect an upgrading within a few years. Guess what, he can also fly an approach, and is quick in understanding a new aircraft type and learning new procedures. He'll go chasing cabin attendants, and stay well out of the way from the offices and managers.
In the end, the company is interested in the better financial bargain, which they get from their statistics. And that's all there is to it.