Here's why:
* Adverbs modify verbs, adjectives, or other adverbs. They answer questions like "how?", "when?", "where?", or "to what extent?".
* Adjectives describe nouns or pronouns. They answer questions like "which?", "what kind?", or "how many?".
Example:
* Oldest is used to describe a noun (e.g., "He is the oldest child in the family"). It indicates the highest degree of age among a group.
Therefore, "oldest" functions as an adjective, not an adverb.