1. Explain it simply
多 + adjective asks the degree: 多大 (how old), 多高 (how tall), 多长 (how long).
多 goes BEFORE the adjective.
The answer states a specific amount (十八岁, 一米八).
2. Show examples
你今年多大?
Nǐ jīnnián duō dà?
How old are you this year?
多大 = how old
他多高?
Tā duō gāo?
How tall is he?
多高 = how tall
你儿子多大了?
Nǐ érzi duō dà le?
How old is your son now?
with 了
3. Share a tip
多 goes before the adjective (多高, not 高多). Use 多 for degree; use 几 to count things.

