They can ask to see if you’re too young. They simply can’t discriminate if you’re too old. Because young people can’t vote but old people vote religiously, so old people get the protections from lawmakers. But they can absolutely discriminate as long as they aren’t blatant about it, because proving it is virtually impossible.
Same way they aren’t allowed to discriminate based on marital status, but they can look to see if you’re wearing a ring during your interview. That one is particularly common in jobs that require a lot of travel; Interviewers will look for a ring, and almost never hire anyone who is married. Because if you have a family, you’re not going to last in the job as long as someone who is single with no local ties. Alternatively, an old tip for interviewing in office jobs is to wear a ring to the interview even if you’re single. Because an office position is more likely going to be looking for someone stable and needing to support a family.