Ah, just found it:
Professional and Age Group wetsuit cut-off temperatures:
Professional athletes will be allowed to wear wetsuits in water temperatures up to and including 71.5F/21.9C. Age group athletes will be allowed to wear wetsuits in water temperatures up to and including 76.1F/24.5C. (Please note that for our U.S.A. Triathlon sanctioned events IRONMAN reserves the right to allow athletes to participate in the non-competitive wetsuit wave for water temperatures 76.2F/24.55C and up to 83.8F/28.8C)
Originally from:
http://www.ironman.com/triathlon/new...#ixzz3lzbX388n