It’s seems like only yesterday Bill Clinton was sworn in as the second-youngest elected president ever at 46 (John F. Kennedy was 43, and Teddy Roosevelt assumed office at 42 after an assassination). On Sunday, he celebrates his 64th birthday in Water Mill at the home of longtime aide Doug and Lily Band. Secretary of State Hillary Rodham Clinton is cohosting. Jason Weiner, of Almond, is catering.