It was black and white - maybe from 30's or 40's - it was a bit of a comedy - a man posing as a doctor visits and makes friends with old people in a retirement home - he seems young and fit for his age, when the residents query it he says he has special pills that keep him young - he gives some pills to some of the residents who do perk up and act younger - some grumpy old resident who doesn't like the new bloke getting the attention of the ladies there digs around and finds out that the new bloke is a fraud - the pills are simply a placebo - the moral was that when they thought they were taking youth pills they acted younger and felt better and happier. It was a clever and funny movie - I saw it as a midday movie on TV some years ago but forgot the title. Can anyone help?

Mr. Belvedere Rings the Bell (1951) starring Clifton Webb

TCM synopsis excerpts (slightly edited):
Famed author and genius Lynn Belvedere is walking in the park when he overhears four elderly people complain about their bleak lives. Belvedere, whose current lecture tour is on the topic "How to Be Young Through Eighty", is intrigued by the idea of entering the home where the oldsters live and helping them to regain their vigor and enjoyment of life. Telling his manager that he will join him later, Belvedere goes to see Bishop Daniels, nominal head of the home, who assumes that Belvedere is Oliver Erwenter, who recently applied for admission to the home. Belvedere does not correct him. His youthful appearance bewilders the bishop, as Erwenter's birth certificate states that he is 77 years old, but Belvedere talks his way into being accepted.. . As Belvedere settles into his new quarters, his lively manner intrigues the residents, and he tells them that his youthful appearance is due to a secret Tibetian potion.. . Belvedere prepares some placebos at the pharmacy and has them mailed to himself at the home.

Mr. Belvedere Rings the Bell (1951) - Overview - TCM.com

There are two other Belvedere movies: "Sitting Pretty" (1948) and "Mr. Belvedere Goes to College" (1949).