HAHA I WAS RIGHT. When they called him 9 and it felt way off, I said "years are 1.7x slower in isekai world so he's like 15."
Going by a standard 18 for being an adult at like age 11, this gives a 1.63x time scale.
We could also say 20 and 12, estimating a higher age since it's been a while as well as Japanese age standards, but I don't think that makes as much sense.
Still 1.66x