At least you're being honest. Look, with all due respect to office workers (I've been one myself), it's often not actually that difficult to train new people on the skills involved. It's not uncommon for a team of ten workers to have two really old dinosaurs who have been doing it for a very long time and are there for trouble shooting and passing on know-how, four people who have been doing it for 5-10 years and they can manage themselves because they know what to do, and four people who are really young or really inexperienced but they have the potential to pick it up and learn.
And if it's a team that handles top secret documents, the only difficult part is managing the background checks - which is actually EASIER with young people who don't have much of a background to check; if you check your search engine for "how hard is it to get Top Secret clearance", you'll see all kinds of stories of people saying it's really no sweat at all, especially compared to higher clearances like SCI (I don't even know why we call it "top" secret, if there are higher levels then it's not at the top) - and enforcing discipline on the rules that young people tend to break, which is more about the threat of punishment than about preventing young people from doing that job in the first place. Out of the four young people in a typical team of ten, two of them will probably get bored of the job and quit within a year, one will probably be fired for breaking the rules, and one will prove really trustworthy and passionate about keeping the job.
So, yeah. It DOESN'T surprise me that 21 year olds have Top Secret clearance. It's more to do with what kind of work their team does than about anything else. But I bet you that almost none of them would have SCI clearance.