Could've all been in a pile. Or maybe they were moving and she lifted his mattress and OH THE HORROR!
Better question: What constitutes a young age? Given that they're only a year apart, provided Sven started masturbating at the average age (11, allegedly), and assuming Sven utilizes the same sock for about a while, you'd have to assume he'd have been doing it for around a year by the time his mom could find 30 socks under his bed. That makes Dora around 11, which is around the time I started sex ed in school. Even then I didn't learn anything I didn't already know by the age of 9. That means that if that's how Dora learned, odds are she actually learned later than most people I know.
Also, if she found 30 socks under his bed, why would she complain about the laundry? He clearly wasn't putting them back into the rotation.
Between those two things and the number of people who still haven't got the clitoris joke, I'm not sure today's comic was one of Jeph's best. Then again, it says a lot about his ability when at his worst he's still very good.