I want to know which one is correct ? "New" is age and "Difficult" is observation so it should be difficult new question but I think it's not ! why?
There are two types of adjective groups: coordinate and cumulative. Coordinate adjectives all equally describe the noun, are always separated by commas, can be rearranged without changing the meaning of the sentence, and an "and" can be substituted for the separating comma. Cumulative adjectives are not separated by commas, you cannot put an "and" between them, and changing their order is either impossible or modifies the meaning.
Sometimes, it is clear what type the adjectives are. For example, "large, heavy book" is most certainly a case of coordinate adjectives, while "expensive custom-made computer" or "large red book" are clearly groups of cumulative adjectives. Sometimes, as in the case with your example, it is not definitive; your example can be either. Because you are not including a comma or "and" in your examples, I assume the adjectives in question are cumulative.
In case of cumulative adjectives, the one before the noun modifies it and pairs with it as a lexical unit while the one before the adjective / noun unit modifies the whole unit. In this case, the meaning is as follows.
new difficult question
This is unusual but possible. This would be used to discuss a new one in a series of difficult questions, such as would be put forward for a discussion at a Difficult Question Committee. The "difficult" here applies to the one being asked as well as all previous questions.
difficult new question
This would be used when a new question has arisen which, in addition, is difficult. "Difficult" applies to the "new question" only and does not necessarily--most probably doesn't--apply to all of the previous questions. We could say, "This is a difficult new question which there should be a separate meeting to discuss."
difficult, new question or new, difficult question or new and difficult question are all examples of coordinate adjectives, are all acceptable, and all mean the same thing.