Final answer:
The word "cuántos" is used instead of "cuántas" when asking for someone's age because the word that follows it, "años", is masculine and plural.
Step-by-step explanation:
The word "cuántos" is used instead of "cuántas" when asking for someone's age because the word that follows it, "años", is masculine and plural. In Spanish, nouns and adjectives have different forms depending on their gender and number. "Años" is a masculine noun and when paired with "cuántos", the adjective must also be masculine and plural. Therefore, the correct way to ask for someone's age is "¿Cuántos años tienes?"
Learn more about Gender and number agreement in Spanish