Welcome to Pandora Hearts Answers. What would you like to know?

Ada was in fact 18 during the main part of the series, while it wasn't explicitly stated in the series, it was stated in the guidebooks, and even in the Caucus Race novels (which Jun has confirmed as canon) that Ada was a sixth-year student at Lutwidge Academy, which made her 18 alongside the likes of Gerald and Josephine. Furthermore, Ada was also confirmed to be 8 when Oz was dropped into the Abyss, add 10 years onto that and she's 18.