"She is not as young as she looks."
Translation:Ella no es tan joven como parece.
http://spanish.about.com/od/adjectives/a/tan_tanto.htm Looks like it's related to the use of the word "tan" as part of the comparison., i.e. the sentence structure is "... tan ___ como ..." Unfortunately Duolingo doesn't teach us that at any point.
I think it might be the sense in which the different words are used. Even though MIRAR, BUSCAR, OBSERVAR, PARECAR can all mean "to look" in one form or another, I think their usage in Spanish differs. I think MIRAR is used more as in "to look at"", BUSCAR is used in the sense of "to search for", OBSERVAR "to observe or notice", and in this case PARECAR means "to appear or seem like". At least, that is the sense I get going through these lessons. I think the more direct translation of this Spanish phrase would be "She is not as young as she appears" or "She is not as young as she seems."
It does seem simpler in English that one word "look" can be used to cover all these situations, but I'm sure that there are advantages to the Spanish system, maybe in clarity...
So, "She walks as she looks." would be "Ella camina como ella mira." Uses the same english phrase, but in Spanish is the meaning is actually more clear.