21 seasons under his belt — but he nearly quit the show shortly after starting. In his new memoir, “When I Was Your Age: Life Lessons, Funny Stories & Questionable Parenting Advice From a Professional Clown,” the comedian, 45, confessed that his “SNL” beginnings were “marred by a ton of rookie mistakes.”When Thompson started on “SNL” in 2003, his confidence “sunk to an all-time low” when he “tanked” a sketch written by T.