The words preadolescence, preadolescents sound the same but have different meanings and spellings. Why do preadolescence, preadolescents sound the same even though they are completely different words?
The answer is simple: preadolescence, preadolescents are homophones of the English language.
The period of childhood just before the onset of puberty, often designated as between the ages of 10 and 12 in girls and 11 and 13 in boys.
Plural form of preadolescent.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition, from Wiktionary, Creative Commons Attribution/Share-Alike License and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").