Drawing styles are partially why. There's a lot of stylization in order to allow quick and consistent renderings of faces while keeping expressions intact. Heads are large for more space, eyes are big to display emotion, mouths/noses are minimized for speed.* Compared to a real adult, heads are disproportionately large with outsized eyes and small mouths/noses. Real children show the proportional differences from adults, though in less exaggerated form. Also, a lot of anime is targeted towards younger audiences. Character designs reflect this.
Physical differences are another reason. Asians in general tend to display diminished secondary sexual characteristics compared to their Western counterparts. Less body hair and muscle mass for men, less breasts and buttocks for women. Asians also tend to look younger than their Western counterparts when judged by Western standards. Because of this, many 17-24 Asian females do have builds not too dissimilar from a 12 year old boy with a bit of a gynecomastia problem. Toss this and the aforementioned drawing styles together and yeah, you're going to get quite a bit youngish looking characters.
* Japanese put more emphasis on the eyes when reading expressions, while Americans focus more on the mouth. So, eyes are expanded, mouths are shrunk.
