Examples of young people playing older people in movies
Jenifer Lewis pretty much made a career out of being a thirtysomething playing a fiftysomething back in the early-mid 90's. She even played a Grandmother in The Preacher's Wife when she wasn't even 40 yet!
Angela Lansbury also played women in at least their 50's a lot back in the early 60's, when she was in her mid-late 30's.
There's also 38 year old Maureen Stapleton playing a 50-60-something naggy Mother in Bye Bye Birdie.
Speaking of Dick Van Dyke, he not only played Bert but also played Mr. Dawes Sr., who was depicted as a really old man, in Mary Poppins.
Guy Pearce also played an old man in Prometheus.