I remember reading about Obi-Wan, it says if youngling's aren't selected to be Padawans by age 13, they don't get trained to be Jedi, they get transferred to other services of the Jedi Core. And when Qui-Gon passed by, I guess what happen was Obi-Wan wanted to prove he could be a Padawan but got dueled his rival with so much anger that Qui-Gon thought he was too reckless. When Obi-Wan turned 13, he was transferred. Later on the same planet I guess, Qui-Gon was in trouble, and Obi-Wan who happened to be there saved him, and Qui-Gon decided to take Obi-Wan as his Padawan afterall. So if Ahsoka was 14 at the start of the Clone Wars, why wasn't she transferred to other services a year ago, instead she's still at the temple another year and becomes a Padawan a year after her Youngling time expired.