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'
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.
This may be slightly tangential, but there exists a common-enough case where elderly people, in response to questions about there age, reply "I'm 70 (for example) years young". This is purely for irony, however, and not in line with standard usage patterns.
There are many words for quantities that express "a lot" versus "a little". There's the example you give: "young", i.e. few years, versus "old", i.e. many years. There are "near" and "far", "short" and "tall", "weak" and "strong", etc. In general, we use the word for "a lot" when asking where something falls on a range. So we generally ask, "How tall are you?", not "How short are you?" We ask, "How far is it?", not "How near is it?" Etc.
We usually use the "few" word only when we want to emphasize the "few-ness" of it.
Whether this is positive or negative depends on the quantity being measured and the context. If I'm planning to run an errand, far away is probably bad. If I'm asking the distance from my home to a toxic waste dump, far away is probably good.
As an old man, I find your implication that "old" is somehow negative offensive. I think that as I get older I am gaining maturity and wisdom. :-)
But seriously, if you ask a small child how old he is, he tries to overstate it. "I'm 5 1/2". "I'm almost 6." Etc. He sees being older as a positive thing. If you ask a middle-aged woman how old she is, she tends to understate it. "I'm ever so much more than 20", as Wendy says to Peter Pan. She sees being older as a negative thing. Old milk is bad; old wine is good. It all depends.
Yes, it would be nice to ask this, isn't it? But, as a fact of life, with each passing day and year, we grow older, not younger. So, at any given time, we are older than what we were at any time before. And if we take "any time before" the starting time of our life, i.e. 0, then the current "oldness" is the years we have lived. Hence, asking "how old are you" to know our relative age from the time of birth.
On the other hand, if we ask "how young are you," it may mean "how many more years are you to live" which is not very polite.
However, for creativity's sake, or for fun, we can ask people "how young are you" just to surprise them with a smile, because they will just think that we are referring to their age as their youth.