Neither young nor old is an advantage to actually knowing the field. It's the training and experience. I've talked to idiots that think their hard drive with drive C: doesn't have any partitions on it. I've dealt with a guy in his 90s that traded traded overclocking tips with me. I myself have solved issues in minutes that a bunch of hot shot college kids couldn't figure out in half a day because they only knew the computer from the gui level and didn't even know what machine language was!
Nobody is ever born knowing anything about computers. You learn it as you go. Older may have had more time, but that's not a guarantee by any means. Then again, the industry is biased against age.
Sometimes it's because of the flawed idea that the young are more enthusiastic and know the new hip stuff. Anyone can be just as enthusiastic and knowledgeable in the new stuff, but after a while you stop jumping up and down screaming cool every ten seconds despite the fact that this cool new thing makes you feel like the first time you were kissed by your true love.
Sometimes they think the old can't change. When often the old are the ones that built the very things they want. Other times it's because this new idea is an old idea in different colors. Some things are never going to be good, but if you don't have a sufficient foundation you probably won't know that in the first place. A great example of a failure that keeps reoccurring about every decade or so is some form of Smellovision. The same problem tanks it every time, no good way to clear out the old smells. Yet about every 10-15 years, someone tries to push their 'new invention'. There's a ton of these same kind of things in every field, but due to the bias for new people in the computer field, they have a shortage of people with the experience to recognize these things.
Often the people doing the supervising or hiring of computer people, aren't themselves computer people and so fall for the myths that are out there.
But I still think one of the reasons that the industry won't admit to, but is still a major factor, is simply that they know the older and more experienced people just won't take the same kind of labour abuse the young ones in their ignorance will. Got a show stopper bug 2 weeks before advertised launch day? Bosses panic and scream for everyone to stay in the office and work around the clock till it's fixed. Older guy tells him to chill out and get a goods night rest, and if they have to, the date can be pushed back. You see, he already knows that allnighters do a lot of work, just not a lot of good work. The problem is more likely to be solved by people that aren't dead on their feet and barely able to function, and that marketing dates are about as trustworthy as the marketing weasels themselves. On top of that, there's a real tendency for those nobody goes home things to violate lots of labor laws, and they often short you on your overtime pay for it anyway, so it's not worth killing yourself over somebody elses mistakes. (In my personal experience, the round the clock panic will often go for 30-40 hours, then somebody who actually went home and got a goods night rest comes in and comes up with a good solution in a few hours. Makes all that panic and crunch time seem kind of pointless.)
Oh well, just my opinion, everyone has their own. :)