A child hears frequencies from 20 Hz to 20 000 Hz; an older person only up to 12 000 Hz.

Step 1. $20\ \text{Hz}$: $\lambda = \dfrac{340\ \text{m/s}}{20\ \text{Hz}} = 17\ \text{m}$.

Step 2. $12\,000\ \text{Hz}$: $\lambda = \dfrac{340\ \text{m/s}}{12\,000\ \text{Hz}} = 0.0283\ \text{m}$, about $2.8\ \text{cm}$.

Step 3. $20\,000\ \text{Hz}$: $\lambda = 0.017\ \text{m}$, that is $1.7\ \text{cm}$.