User blog comment:BlueBarracudas555/Ridonculous Race Contestants Ages/@comment-1416490-20150807215144

For the time line I always guessed from the US airdates of what the year setting for the show might be.

2008 - TDI and TDA 2009 - Celbrity Manhunt and TDWT 2010 - TDRI 2011 - TDAS, TDPI, and TDRR

So the original cast would be born around 1992 to early 1993, the second cast would be late 1993 to early 1994, and the third cast would be between late 1994 to early 1995. Again this just all guessing since we don't even know their birth years, but we can easily figure them out.