President Joe Biden’s 79th birthday has renewed questions about his physical health, mental acuity, and succession plans as Vice President Kamala Harris, his heir apparent, comes under even more scrutiny.

Aides half Biden’s age have struggled in the White House’s pressure cooker work environment, according to Cesar Conda, a Republican strategist and former Vice President Dick Cheney’s onetime domestic policy adviser.

“That’s why staffers last only one to two years,” Conda said of “the incredibly high-stress” workplace.