"Joven" (English: "young man/woman") is an okay term for both male and female young adults (around 20-25 years old), as long as you remember to use the correct determinant: "el/un joven" for a young man, and "la/una joven" for a young woman.
El joven desea algo para beber.
The young man wants something to drink.
La joven desea algo para beber.
The young woman wants something to drink.
Care should be taken when "joven" is used without a determinant or another word that makes its grammatical gender explicit (such as an adjective that is declined for gender, like "estimado/a", but unlike "inteligente"), since it could by default be interpreted as being masculine. In order to avoid confusion, "señorita" (English: "miss") could be used for young women.
Joven, ¿desea algo para beber? (Often used for males only.)
/woman, do you want something to drink?
Señorita, ¿desea algo para beber? (Feminine only.)
Young woman, do you want something to drink?
When dealing for the first time with anyone past 30 years old, I would call them "señor" (English: "mister") or "señora" (English: "mistress", without the connotation of being a married man's lover), never "joven" or "señorita", which could be interpreted as if I were questioning their maturity or otherwise belittling them.
Señor [<apellido>], ¿desea algo de beber?
[Mr. <last name>,] Do you want something to drink?
Señora [<apellido>], ¿desea algo de beber?
[Mrs. <last name>,] Do you want something to drink?
However, sometimes unmarried women, regardless of their age, take offense at being called "señora", and would rather be called "señorita". There are two reasons behind this:
The term "señora" is sometimes interpreted as an insinuation that the woman in question is old.
Especially among the elderly, the term "señora" is used to refer to someone's woman/wife. Since traditional Catholic values are deeply-rooted in most Spanish-speaking societies, insinuating an unmarried woman has already been "taken" by someone could be taken as an insult.
The reply usually given by women who take offense at being called "señora" is:
[Are you calling me a] Mistress? [I am a] Miss!
Rarely do males take issue with being called "señor", as the term by itself does not carry an "old man" connotation. Using "señor" is correct, even when referring to males who keep a youthful spirit in spite of their advanced physical age:
El señor sabe cómo mantenerse joven.
The man knows how to remain youthful.
La señora sabe cómo mantenerse joven.
The woman knows how to remain youthful.