Numbers above 60 (예순) are not commonly found as native Korean numerals, although they are defined. After 99 (아흔아홉) though, only the Chinese set (백, 천, 만, …) is available.
Aren't certain things typically specified in Korean numbers, while others use Chinese numbers? If that's the case, what happens when the former go above 100?
IIRC, age typically uses Korean numbers, so what happens when someone turns 100? Does the number system suddenly change?
yes you should change to the other number system. Originally, 온 was 100 and 즈믄 was 1000 but it died out.
I read somewhere that sometimes these systems are mixed. People sometimes say things like 백스물, or 백다섯, for example. Anyway, it's neither formal nor elegant nor really correct, I presume.
Yes, exactly. Everything what you count in native Korean numbers after reaching 99 you have to change to Sino Korean numbers.