User blog comment:BlueBarracudas555/Ridonculous Race Contestants Ages/@comment-4389268-20151014132431/@comment-26920609-20151015011650

They weren't incorrectly aged...

They were born in the early fall, so:

16 in the summer Island and Action take place in, and turn 17 almost immediately after TDA

17 in World Tour and Revenge of the Island, and turn 18 almost immediately after TDRI

18 in All-Stars and Pahkitew Island, and turn 19 almost immediately after or during TDPI

19 in The Ridonculous Race.

So with this, it's:

Early-Mid Summer 2007: Island

Mid-Late Summer 2007: Action

Spring 2008: World Tour

Early-Mid Summer 2008: Revenge of the Island

Early Summer 2009: Leonard turns 16

Mid Summer 2009: All-Stars

Late Summer/Early Fall 2009: Pahkitew Island

Earyl Fall: They turn 19

Mid-Late Fall 2009: The Ridonculous Race

Christine confirmed this too. That's why Leonard is still 16, his birthday is almost immediately before when TDPI takes place