What are the usage rules for nino/nina vs. muchacho/muchacha?

Question

What are the usage rules for niño/niña vs. muchacho/muchacha? What are some specific cases or examples of when each should be used? How can you tell which should be used?

Answer 1

I would add that muchacho would include teenagers.

Answer 2

While I would agree that niño = boy and niña = girl refer to children and muchacho/a can refer to older children the last two words are much less common than they used to be among los jovenes / younger Spanish speaking natives Chico=boy y chica = girl are more common nowadays

I am open to correction on this

Answer 3

Niño y niña refer to children: little boy or little girl. Muchacho(a) usually refer to younger people but not children. Young adults. I think of muchacho and muchacha as Guy or Girl in his/her 20's and 30's.

Anyone else have any input?