It would be naive to suggest that ageism doesn't exist. It clearly does. The trouble is that it's so hard to prove that you've been a victim of it that there's not much you can do in most cases. If asked why the other guy was chosen ahead of you, no employer is going to say it's because he was younger, even if that was their real motive. That said, I do think the situation is better than it has been historically, particularly in large organisations like airlines. With a set process and scoring system that includes a variety of different decision makers you're much less likely to become a victim of age discrimination.
There are probably various reasons fewer older people make it through airline cadet recruitment processes. However, I imagine the most significant factor is that far fewer of them apply in the first place. When you reach your mid to late thirties there's a good chance you've got a mortgage, a husband or wife, kids, a family dog, and a half decent job. To drop it all and undertake a new career as difficult and expensive as aviation can be tricky to say the least, which is why the ranks of ab-initio applicants are predominantly made up of guys and girls in their late teens or early twenties. Clearly some people break this mould though. I know of guys in their mid thirties to mid forties who have applied for airline programmes and made it through.
A lot of posts above carry sound advice in my opinion. Usually people following their dreams look for information that supports what they want to know/do, rather than objective points of view that assess the facts. As hard as it is to accept, sometimes career aspirations never become a reality, and you can spend your whole life chasing the dream and end up with neither the dream nor a life worth living. My advice has always been to never risk what you can't afford to lose; and that applies to relationships as well as money.