Shirt sizes for Japanese teenagers

I'm planning to buy some T-shirts for average sized Japanese female teens about 18-21 years old. Probably will be buying from USA.

What sizes (in S/M/L or inches etc) would I need to be looking at for?

While I'm here, what would be suitable sizes for boys of same age?

Nobody..?

No answer Tom because Japanese are like everyone else..they come in all sizes. Both young men and women do tend to be slimmer--often much slimmer-than North Americans, but they can be short, tall to very tall so it is hard to tell you what to buy.
Do you have anyone in mind or are they random gifts for people you may meet??

I am 5' 7", slim waist--compared to North Americans, but somewhat muscular torso and, depending where the T Shirts or shirts come from, can wear S, M, or L !!! my problem are with the sleeves length..

The average teenage girl size is 7 or 9 in Japan, but I'm too stubburn to figure out what that is in the U.S. Same for boys. They're usually M or L in Japan, but not sure what that is in the U.S. When buying souveniors, a lot of people buy big sizes of loose designs so that it can fit anyone.