"Skills out of date" is suspect, at best. We have been putting all new languages and libraries on top of the same old computer science concepts for decades now. It wouldn't be that hard to age-discriminate by looking specifically for Clojure experience, while saying anyone trying to substitute ANSI Common Lisp experience has their "skills out of date".
To put this into analogy form, it would be as though mathematicians could be perceived as out of date because they still used "x + y" for addition, rather than "add( x, y )", or "push(x) push(y) add", or whatever the latest fad in mathematical notation happened to be at the time. The concept of addition (over real numbers) hasn't changed for centuries. Some skills just can't go out of date. And yet some people are viewing those skills through a lens that makes older candidates look worse on paper, and are blaming the skill set, rather than the lens.
There are some skills that make otherwise inexperienced people more productive when they all share those skills, but at some level it all comes back to screening based on experience rather than aptitude. I don't have time to learn every new skill that comes down the pipe, from mere speculation that a future job opportunity might require it. I'm more likely to pick up only those that would be useful in my current day job, or for my latest side-project. All some employer has to do is pick a lot of "new" skills, that aren't appreciably different from new names for old skills, and magically, my skills are no longer up to date.
I hypothesize that the age discrimination in the industry isn't based on age, directly, but rather on perceived costs of employment, which tend to rise as people progress through their careers. Old people are more expensive. They want higher salaries. A lot of them have families, and mortgages, and their health insurance costs are higher. They worry about their 401(k) accounts.
One would think that it's easy to screen by age, based on the length of the resume, and simple math on any of the dates on it, but the people screening the resumes, and the ones rejecting candidates post-interview are not necessarily the same people. The HR folks know that age-related discrimination is illegal, so may be structuring sham interviews so that audits after the fact don't leave the company liable for enforcement actions. I have been through a few interviews where it just seemed like everyone was going through the motions. Maybe they already picked someone and were using me to cover their ass? Maybe there was something about me in particular that prompted an immediate rejection? Maybe I didn't fulfill some secret requirement? Did my breath stink? Did I whiteboard that wrong? Nope--they found someone to do the job more cheaply, and they were shopping around to make sure they got the best price. It just so happens that youth correlates positively with cheap.
It's difficult to say, when no one ever gives any interview feedback whatsoever, positive or negative. When you're in it, you start to get paranoid.