"These students are young."
Translation:Bu öğrenciler genç.
You can only mark the plural on bu, şu and o (becoming bunlar, şunlar and onlar) when these words are used as pronouns, not when they are used as determiners/adjectives because the plural ending appears on the following noun.
Bu öğrenciler genç. = These students are young. bu genç öğrenciler = these young students
Bunlar genç. = These (ones) are young. Bunlar genç öğrenciler. = These (ones) are young students.
A little tip would be, if you can add "ones" in English after these words, you can add the plural ending. You can't say "these ones students" to mean "these students", so you can't use "bunlar" there.