Ichard told his son, " (your, you're) young, but when you grow up, you'll understand."

Every time you see you're, you should automatically know it's the joining of 2 words. Instead of reading you're, you should immediately think: "You are"

So, read it by saying you are.

Ichard told his song, you are young, but when you grow up, you'll understand (same with you'll, you'll is you will)

You're is correct.

Your is generally used as a possession term, normally with an item.