Who was the oldest contestant on 'The Amazing Race'; Don from season 6 or Meredith from season 7?

Don - 69
http://www.cbs.com/primetime/amazing_race6/
Meredith - 69
http://www.cbs.com/primetime/amazing_race7/

So unless you can get their birthdates both were 69 when they did the race. Gretchen was 66 and Mary was also 66. So both teams were the same age.

Both were 69.

Meredith was 69, Gretchen was 66.
http://www.cbs.com/primetime/amazing_race7/teams/meredith/

Although they are the oldest couple on this Race, Donald, 69, and Mary Jean, 65, believe that their life experience and appetite for adventure will be a key asset in winning The Amazing Race.
http://www.cbs.com/primetime/amazing_race6/teams/don/bio.shtml

My reference for Don and Mary Jean gives Mary Jean's age as both 65 and 66.

Meredith Smith and Gretchen Smith, married grandparents and retirees from Easton, Maryland. The show's all-time oldest team, they serve as this season's sacrificial lambs. Meredith is a 69-year-old retired executive while Gretchen is a 66-year-old retired flight attendant and registered nurse.

http://www.realitytvworld.com/news/cbs-reveals-identities-of-the-amazing-race-7-contestants-3237.php