An interesting question, crashandburn.
I suspect there are several reasons for this. The market currently has more pilots than vacancies so the companies will take the "cream" off the top. Don't worry, when the industry turns round (and I believe it will looking at past performance and current projections for air travel), the companies will start reverting to the "bums on seat" policy - i.e. if you have a licence we'll employ you!
Please remember that all companies are now looking at nothing but the bottom line. If you are older you are perceived to have more baggage (who knows, you might even have a family!) and may reach the point of no return (loss of licence due medical etc) before those who are younger. It is also perceived that you might have more difficulty completing training and/or reaching the required standard for command etc. All of this is, I believe, quite shortsighted but that is how most of the hierachy running the companies see things.
However what is more ominous is that it is presumed that younger staff are more likely to "toe the party line" and not "rock the boat" when it comes to things like pay and conditions and, dare I use the term, "worker rights"! If this is one of your first jobs in the employment market and you have spent lots of money getting your licences then you don't want to risk putting yourself in a position where your job is on the line. Perhaps if you are older you might have paid off a fair bit of the mortgage and even have a partner/wife who also is an earner. Obviously this will all depend on personal circumstances but I am sure you get my drift.
Older staff are more difficult to programme to adopt the company culture and all the other crazy "mission statements" that they all love to spout about these days!
Please do not think I am not on your side - I am! But this is just the way it is!