Dezi looked old for his age.

Well, remember that he had aged some. We tend to "visually" remember him as he looked in his I Love Lucy days. That show began in 1951, so a good 16 years had passed when we saw him resurface in The Mothers-in-Law.

When I first watched the show (several years ago), I remember saying to myself "wow, he looks awful and he has not aged well at all" (with regard to Desi Arnaz).

So, I don't think they made him look older (with make-up, etc.). I think he naturally did look older.

Also, I think I read that he was an alcoholic. And that tends to age people in the looks department, also.

As to your theory: what would be the reason that "they" would want to age him, to make him appear older? The "they" probably refers to the producers and creators. And I believe that those jobs were held by Arnaz himself. So, why would he himself want to make himself look old? For most Hollywood actors -- especially those with big egos (like him) -- that is the last thing that they want. No?

Desi was just one of those people who didn't age "gracefully". In the first season of Mothers-In-Law, he was only 50, but definitely looked 60-65.

Yeah! Just seen the episode where he stays with them, what's up with his teeth??? When he was singing to the ladies... Something (stained) was wrong with teeth.