I've seen many job postings with qualitative descriptions along the lines of: "You're a young, ambitious, fast learner..."

Enforcement is, uh, spotty. But the regulatory environment is enough to give applicants an advantage in this situation. Interviewers know they're ignorant of complex shifting rules, so are afraid to ask anything personal that isn't also spinnably vague.

Easy. Ask questions that only someone who has recently revised for exams would know. No-one enforcing this has any idea that coding a red-black tree from scratch on a whiteboard isn't something that any working programmer actually does...

You could just take your job hunting seriously and do a bit of studying. Though the details of a red-black tree are a bit much to ask anyone.

Generally speaking, there's nobody out there proactively enforcing antidiscrimination laws. The enforcement pathway is reactive. You get penalized for discrimination (in theory) when someone you've discriminated against brings legal action against you.

In this case, someone would have to sue them. For instance, an older developer who interviews, believes he or she is being discriminated against on the basis of age, and uses the advertisement as further evidence in the complaint.