"Her daughters are young."
Translation:Ihre Töchter sind jung.
Because "sie" is "she" or "they", not "her". "She daughters are young" doesn't make sense.
Ihre is "her" or "their" depending on context and is basically the possessive form of "sie". If "sie" owns something, it is "ihre". However even the word "ihre" has different forms I think, and unfortunately, like you, I am also learning and don't know all the forms yet. Es ist nicht einfach, aber ich lerne seit zehn Monaten.
Attributive adjectives (ones before a noun) need an ending that depends on the gender, number, and case of the noun that follows, as well as on the article or other determiner that precedes the adjective.
Predicate adjectives (roughly: those after "to be") do not inflect for gender, number, and case.
Thus you would have junge Töchter and ihre jungen Töchter (both with endings) but ihre Töchter sind jung (without ending).