David Cook reminds me that in North by Northwest, the mother of Cary Grant (who was born in 1904) was played by Jessie Royce Landis (who was born in 1904).
Jim Newman reminds me that in Chitty Chitty Bang Bang, the father of Dick Van Dyke (born: 1925) was played by Lionel Jeffries (born: 1926).
Galen Fott reminds me — well, actually informs me since I didn't know — that in the original New York company of The Fantasticks, 30-year-old Kenneth Nelson played Matt ("The Boy"), while 24-year-old Jerry Orbach played the older, wiser narrator El Gallo.
And Erik Peek, Eric Newsom, Steve Darnall and Alexander Pascover all remind me that in The Manchurian Candidate, Angela Lansbury (born in 1925) played the mother of Laurence Harvey (born in 1928).