Here's how lifetime passes were dealt with at Citi Field last year, as per the "2013 Credentials" posters displayed at various entry points in the ballpark.
Underneath the image of the pass, it says:
Gold Lifetime Passes are subject to individual club policies. In addition, a number of lifetime passes have been issued to veterans of war, prisoners of war and other military personnel. Please honor then when presented, subject to your individual club policies, and/or call the Club Relations Department at Major League Baseball should there be any questions.