This is an illustration of markedness: a cognitive / linguistic phenomenon that is common to many languages, not just English. Some adjective pairs are naturally asymmetrical:
- Young / old
- Short / tall
- Dumb / smart
You would normally ask how tall a person is. On the other hand, if you ask "How short are you?", then it feels unusual, and it's no longer a neutral question: it's implying that you think the person is unusually short.
Similarly, if you ask "How young are you?", then it implies that you think someone is unexpectedly young. If you say that "He is 80 years young", there is meta-information that the speaker thinks that the subject looks or acts younger than 80 years.