Calling this sexism is the weirdest possible interpretation. In most places and cultures across time, teens often took on responsibilities that adults would. In the vast majority of cultures boys we in modernity classify as minors were often treated as men. Generally from the moment they could competently participate in the same duties as men. Sokka at the start of the series is in charge of the defense of The Southern Water Tribe. The women you see there have a responsibility to the children. All of the grown men of fighting age (according to their culture) left to participate in the war. If you have any basic concept of the human condition, you’ll quickly realize that the families of those men don’t just stop existing while they’re an ocean away. They didn’t seem to have the luxury of leaving much in the way of defense for their home and Sokka (this was a clear plot point) was not of fighting age when his father and the rest of the men joined the war. He came of age some time after. So as a young man of his tribe and son of the chief, the safety of the children and wives of his fellow men became his responsibility as the only man left. This isn’t sexism. This is a depiction of the human condition distilled down into a children’s story. It is tradition, born of necessity, which has seen human populations survive even the harshest conditions and most unfortunate of circumstances for most of our existence. Looking at it as mere “sexism” is a luxury afforded to we in the West whose women and children are wrapped in layer upon layer of highly advanced statecraft, weaponry, and technology.