To be 70 years young is sometimes far more cheerful and hopeful than to be 40 years old.

Almost certainly attributable to Oliver Wendell Holmes, Sr., who is, in various sources, credited with having said this in letters to Harriet Beecher Stowe (who turned 70 in 1881) and Julia Ward Howe (who turned 70 in 1889), as well as having made the commented about himself. Holmes, Sr. reached the age of 70 in 1879, while Holmes, Jr. reached that age in 1911, some time after the earliest reports of this quote.