hello, i am a chinese girl.I can answer your question. "少" always means the amount,quantity. while"小" means the size and ages .Maybe that's it

when we say something with small quantity, we use“少" while we say that something with a small size, we use“小”,and when we say someone is young, we can say "小" Both of the two are adjectives, and they are a bit similar in writing, but they are different in meaning.

"少"is very little,few in amontand"小"is very small or in a little size,but some time we can chang betwwen them.

haha 年少 and 年龄小 here they all have the same meaning young

少 baisically means little ,few in amount and 小 means small or little in size. but when the context changes, the meaning can change too.