Bernard Cribbins(1928-2022)

Aged 79 when he made his first appearance as Mott, he is the oldest actor to have played a credited companion on Doctor Who (1963) or Doctor Who (2005).

[on Doctor Who (1963)] I didn't actually audition. But when Jon Pertwee was leaving, producer Barry Letts--who died quite recently--interviewed a lot of actors, one of whom was me. I went along and sat down and he said, "Now, then, what can you do?" I said, "I'm a very good swimmer, I was a paratrooper, I could fight"--and he said, "Oh no, no fighting no, the Doctor is never seen fighting at all!" So Tom Baker became the next Doctor, and one of the first things I remember him doing was knocking somebody out.