Here's a look at the many married couples:

Tim Considine/Mike: Born 1940
Meredith MacRae/Sally: Born 1944

Doris Singleton/Sally's mom: Born 1919
Sebastian Cabot/Sally's dad: Born 1918

Don Grady/Rob: Born 1944
Tina Cole/Katie: Born 1943

Fred MacMurray/Steve: Born 1908
Beverly Garland/Barbara: Born 1926

Stan Livingston/Chip: Born 1950
Ronne Troupe/Polly: Born "circa 1945"

Doris Singleton/Polly's mom: Born 1919
Norm Alden/Polly's dad: Born 1924

Fred MacMurray/Fergus: Born 1908
Anne Francis/Terri: Born 1930

We see that both Ronne Troup and Meredith MacRae were around Don Grady's age, so if things had shaken out differently, they could have been potential brides for Rob...!

Also, Doris Singleton and Sebastian Cabot were certainly very close in age.

Bump

Bringing back this very old thread -- IMDb now does have Ronne Troup's DOB as June 10, 1945.