Quote Originally Posted by Dark Dragon View Post
She's older because the most unique thing about Wendy is the fact that she is the token loli of the guild.

All the other characters had something major about their characteristic flip. Natsu weakness to transportation is reverse, Lucy is the typical female action heroine as oppose to the regular blonde bimbo and Ezra is Lawful Evil as oppose to Lawful Good.

Wendy really doesn't have any other major trait other than being a loli so the only thing Mashima could do is make her older.
that's the best explanation I've heard yet!