Haven't seen numbers
Haven't seen any numbers on life expectancy of lennox-gastaut patients.
Maybe nobody has collected and published that information. One website reported a 3%-7% mortality in patients over a 9 year period (that sounds like 1/2 % per year). The deaths were often due to accidents. In such disorders, if a person is watched and cared for closely, he can outlive parents, and die of other natural causes.