These are  nice well made boxer briefs.  I didn't know what size to order for my 13-year old.  He is smallish and the size L I ordered are too big.  I will keep them as he'll grow into them.

From the package

Size  Age  Waist
XS  2-4  18-21"
S  6-8  21-23"
M  10-12  23-26"
L  14-16  26-28"
XL  18-20  28-30"