I read that a lot of people don't do this because they are afraid of $RANDOM lawsuit.
First, if you don't push back on "young", you're asking for trouble right from the start. You gotta say, "I can't screen based on age, that's going to get both of us in trouble." The second a not-too-bright recruiter says, "I can't hire you, you're too old", everyone loses. So don't get yourself into a position where you're screening for young in the first place. It's fair to ask the hiring manager why they want a young person, and it may simply be that the job isn't something anyone with real experience would tolerate. Then you can describe the job in such a way that it highlights the requirements for someone at that level... "We work hard, day and night, to ship stuff as fast as possible, fueled by Mountain Dew and dreams of glorious stock options."
Once you've craft a job pitch that only someone who wants to work day and night would apply to, if a more experienced person applies to the job, you can reject them by focusing on the culture... "Hey, I don't think you'd be a fit here. Your skills look great, we'd love to have you, but do you really wanna work 70 hours a week? We're an adrenaline fueled sweat shop, and you've been at a cushy B2B SaaS company for a few years. If I'm wrong, let me know!"
Here's an extreme example. I rejected a candidate, and felt bad for him. I gave a specific reason why he'd never get the job. A few hours later, he showed up in the lobby of our office building, coked out of his mind (that's what it looked and felt like, but he was probably just intensely upset). He started demanding to see HR. Shit went from chill to super uncomfortable in 0 seconds flat. I walked up to him, suggested he leave, at which point he realized I'm twice his size, and he departed. Yikes!
I stopped giving invalid rejection explanations after that. It's just not worth it, and that's part of living in the world that sucks more than I'd like it to.
They usually don't. The same reasons apply. In Europe there's less fear about random lawsuits, but there are so many rules about hiring that it's just a minefield with lots of material for very legitimate lawsuits. You didn't get the impression that the applicant is a good culture fit? You better hope his name didn't sound Turkish, or that quickly sounds like unlawful discrimination.