why do people say the term young lady for youth?

A woman who is not far advanced in life. Also a girl.

A form of address used by an adult to a girl, often in anger.

‘You realize, under laws of Tennessee, we are now legally married, young lady.’

‘I've been thinking about this for a long time. No man is an island, young lady.’

dated - A girlfriend.

https://en.oxforddictionaries.com/definition/us/yo...

a young, usually unmarried woman of refinement, grace, etc.

2.

any young woman.

3.

a girlfriend; sweetheart; fiancée.

http://www.dictionary.com/browse/young-lady

a usually unmarried young woman of grace, manners, or distinction

https://www.merriam-webster.com/dictionary/young%2...

A term of endearment or admonishment for a girl.

https://en.wiktionary.org/wiki/young_lady

an expression that older people sometimes use for referring to or speaking to a girl or young woman

(OLD-FASHIONED) a girlfriend

http://www.macmillandictionary.com/dictionary/amer...

A youth is a teenage male(never a girl) A young lady is a female.

it is polite as is young man