I went to school from 7 to 9 and then again from 15 to 18. It depends on the subject but on average the depth should be in the same ballpark. He definitely knows more math and physics than I do, maybe chemistry as well, I think I'm better at history. On languages, he's probably better at German than me (note it's our native language), I'm probably better at English than him because I use it more, I don't know how much high-school French he remembers but I assume not as much as I learned in the last few years, I speak Dutch (well somewhat) he doesn't. We're both not very musically inclined.
With respect to well-roundedness: would you say your knowledge is also covers as much depth as your brother’s?