It should actually be written, "15-year-olds" (with hyphens). This is what we call a compound adjective or compound noun, created when we combine words to create a single adjective/noun (e.g., "one-way street," "state-of-the-art technology," "mother-in-law," etc.). The hyphens show that the words the connect are all acting as a single adjective. In your example, "15-year-olds" does refer to those teens who are 15 years old. With all compound adjectives/nouns, we remove the plural in the individual words when we connect them, so "years" becomes "-year-" (e.g., "a two-foot piece of wood" or "a six-day war").

Brad has a good answer about how to use these compound adjectives and how to write them, with one change: You only put hyphens between the words used as adjectives. "15-year olds". Here "olds" is the noun, so it isn't connected by the hyphen. "15-year-old students" Here "old" is part of the adjective so it is connected by the hyphen. As to your other question, it doesn't mean all 15-year olds, but only that there is more than one of them

it's depend th whole sentence but you can just say "15 years old"