``The Biggest Loser'' celebrates a dozen seasons by welcoming two new trainers to the show: professional tennis player Anna Kournikova and fitness expert Dolvett Quince. Also new to the show, for the first time contestants are grouped by age in what is being dubbed the Battle of the Ages with three teams of five players each. The youngest players are all under 30, the contestants in the middle group are between 30 and 49 and the remaining players make up the 50 and over group (the oldest contestant is 65). Following the first challenge, the teams (in the order in which they finished) choose their trainers -- Kournikova, Quince or returning trainer Bob Harper. As always, the contestant who loses the most weight during the season takes home the $250,000 grand prize.