Oldest film with two living actors?

Hmmm....

Oh, yes.  Especially the one on the bottom right.  She's thinking, "This is, like what, the twentieth take?"

Maria Riva had a small role in The Scarlet Empress (1934) at the age of nine. She's 97 today. 

Yeah, I think The Country Doctor is probably the correct answer. At least when it comes to Hollywood. There are several pre-1936 films with living actors, but they didn't appear in the same ones. For example, Marilyn Knowlden and Cora Sue Collins both worked steadily in the 1930s, yet their paths never crossed in that decade. If you combine their filmographies, it makes for an impressive 1930s list actually. Up until 6 years ago, both of Gamin's younger sisters from Modern Times were alive: Gloria DeHaven and Gloria Delson. I think the latter is still going strong at age 96.

In 1937, Betsy Gay (more famous for her singing than acting) and Sidney Kibrick (Our Gang regular) appeared in Nothing Sacred. However, both were uncredited. Kibrick as a boy chorister and Gay "in a group singing role". IMDb doesn't list it as one of her credits, but the Hillbilly Music page for her does. Next on the list is probably The Adventures of Tom Sawyer in 1938. Gay also appears in it, along with Cora Sue Collins and Margaret Kerry. Collins, of course, had the best role of them, playing Amy Lawrence. Gay was uncredited, but at least had a full name, unlike Kerry. TAoTS is probably the oldest movie with 3 living cast members.

Five of the Kind, released later in 1938, also had 3 living cast members. The Dionnes plus Johnny Russell. And yes, this is the same guy who became U.S. ambassador to Oman during the Reagan administration. His political career means that his death won't go unnoticed, which can be a concern when it comes to not-super-famous actors from 80+ years ago. IMDb has so many of them with a lack of death date, yet that doesn't mean they are still alive.

Infants aside, I'm trying to think of the oldest feature length movie with two cast members that had prominent roles. I can't think of anything older than Meet Me in St. Louis in 1944, with Margaret O'Brien and June Lockhart. Darryl Hickman was also in it, for the record. I got nothing for 1945 and 1946. In 1947, we have the PRC film The Devil on Wheels, with Hickman, Terry Moore & Noreen Nash as leads. I think this is the oldest one where at least two now-living actors were adults (18+) at the time of the release. Nash appeared with Ted Donaldson later the same year in the Western The Red Stallion.

Speaking of Terry Moore, I don't think it gets acknowledged enough how insanely long she has worked in the industry. The longest gap between two releases in films and television since 1940 was 6 years. She has barely taken a break in 82 years, I suspect she is trying to outdo her old friend Mickey Rooney.