More can always be said about this, but I wrote about Pétain's dementia in this previous answer.

To expand a little on this, Pétain was probably in the early stages of dementia when he was appointed in 1940, but he was able to "mask" these symptoms, which were not in any case recognized as such at the time by the medical profession.

By the time of his trial in 1945, his state had worsened and his fragility was in plain sight. His first interrogation session in May 1945 did not go well, with Pétain appearing confused and unable to provide answers to the complex questions of his interrogators. His first defender, Payen, tried to use the "pleading senility" strategy for a while, but Pétain hated this and sometimes showed his disapproval by gesturing. He said after the trial that he had been outraged by Payen's attempts at showing him as senile.

Neither his other defenders nor the prosecution mentioned his mental state, perhaps to spare him humiliation. It was in everyone's interest that Pétain could stand trial and that he could explain his wartime actions. The press generally did not go further that noting that Pétain looked frail, absent-minded or tired, but not abnormally so for a man of his age facing a difficult trial. He usually kept silent, with occasional outbursts or protestations, and that did not seem unusual either. Some even thought that he was a "cunning old fox" (L’Aurore, 2 June 1945) or that he was "faking indifference and senility" (L’Humanité, 24 July 1945).