ABOUT
A preposition is a word (usually a short one) that are used to show a relationship between a noun or a pronoun and other words in a sentence. Prepositions are usually followed by nouns, pronouns or gerund verbs, they can also be found at the end of a sentence. They may express position, direction, movement, possession, time, how something is done.
"About" is a preposition because it is placed in front of a noun "her early years" while connecting a particular action (the singing of the caged bird) to another (her early years).