Tagging content (i.e. categorizing content using "tags") is a sensible way to group content on a web site, but does tagging actually affect SEO?

From an SEO standpoint, how does tagging content affect search ranking? Is there any way to optimize the set of tags for SEO? Should I limit the number of tags in any way?


Question: what age are you
Answer: 
  Is the question of "what age are you" idiomatic,common or does it
  sound naturally in the meaning of " how old are you "
  
  Can we say this structure could be used for materials as " how age is
  it "


"What age are you?" is definitely not idiomatic.  Using it would mark you instantly as a non-native speaker.  However, it makes logical sense and it would be understood.

"How age is it?" would be even worse.  While most native speakers could likely figure out what this meant, it sounds simply wrong.

If you were asking a person about his or her age, you'd almost always say "How old are you?" or "What year were you born?" or "When you were born?" -- with the first one being by far the most common.

If you were asking about an object, you would usually say "How old is it?"  But you might also say "When was it built?" (in reference to a building or structure) or "How long has it been here?" or "How many years has it been here?" or "How long has it been around?" -- again, the simple "How old is it?" would be most common.