terminology - Why how 'old' are you, not how 'young' are you?

English (the language) always gives an impression of being positive. For example, when little kids are making mistake, it will refer to as 'being creative' instead of 'being incorrect'. So as my title, isn't it the opposite? I assume asking how "old" is not very polite, so I usually avoid it and ask 'what's your age'

Answer

English (the language) always gives an impression of being positive.

This is not really a property of the language per se as the surrounding culture in which it exists. It is still very if not equally important to understand this if you want to effectively communicate in a given culture. Read about political correctness.

Why how 'old' are you, not how 'young' are you?

You don't gain youth as time passes, you gain "oldness." So the oldness is what's measured. Measuring something you don't collect outside of a scientific context is weird. It's similar to asking "how much air is in this cup" as you are pouring water into it - you don't really want to know how much air is in the cup, but how much water.

I assume asking how "old" is not very polite

I would say in the US at least - it isn't unless among family, people who know each other well, or authority contexts where age verification is called for, such a police offer asking the age of a teenager who is doing something they shouldn't. If you are older than the person and taking on a mentor role, it should be OK to ask. Another permissible context for asking age is collecting details on people registering, signing up for something, or attending an event.

But asking "how young are you" doesn't make it polite, it makes it seem like you are trying to deliberately not use the word "old". I would avoid this if you are trying to be polite.