Depends on the job and on the qualifications required to do that job. I work at a fitness facility in the front office and I typically can't stand dealing with an employee who is not computer-literate. Generally, when it comes to age I don't hire over the age of thirty because most who are over that age are absolutely fucking clueless when it comes to computers (and our sales module is simple as it gets). Of the two I have hired over that age (one 43 and one well into her late fifties), they both fucked up paperwork and sales transactions worse then the average 16-year old with NO experience. When it came to physical labor, there was also a division there as well. We don't have much to do, but I do not hire someone who cannot move equipment well and isn't able-bodied enough to do the requisite daily cleaning tasks that the job requires. But if they're able to do all of the job tasks, I really don't care either way how old they are, just whether or not they can do the job with efficiency and regularity.
Some employers may be different, though. I worked for the government on battleships for a while and they have a typical system in place where the young, cheap, and spry youngsters are generally saddled with the old, fat, out-of-shape and generally immobile sloths who've been there a while. The sad thing is that because the guy I was with had much longer tenure than I, he made nearly $30 per hour for sitting on his ass and basically directing traffic. Meanwhile, I scraped $11.00 per hour while doing all of the physical shit-work. In that case, I think that some qualifications have to physically be met because I can't stand seeing someone bust ass on the same job and get mediocre pay while a lazy fucker rakes in the dough.