niño y niña are used for little girl and little boy, let's say until they are 14 years old Chica y chico are used to talk about from teenagers till young adults. Muchacho y muchacha mean the same as chico y chica. However, I never use the words "muchacho y muchacha" and I always use chico y chica. I think to use muchacha or to use chica that might depend on the country.

The main difference between these nouns is the age range that they kinda imply niña/niño for children chica/chico for teenagers (or preteens, and maybe young adults) muchacha/muchacho for young adults (it can also be used with teenagers)

chico(a) is young niño (a) is kid muchacho is same chico(o)

It's a gender difference. Usually if a noun ends in "a" it is feminine, bit if it ends in"o" it is usually male. The same concept applies to you r examples listed above.

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