pretty sure other people have been convicted of crimes before. he is fantastically old though. maybe he got sent to juvie back in his childhood hometown of Bedrock and was the first convicted criminal!
Technically, President Grant got pulled over, and taken into custody, in 1872 for "speeding on a horse within the city limits of Washington DC." It was the third such ticket he was given in his life, the first two being in the early part of the 1860s when he was just a General. The cop tried to let him go, and Grant cited section 1983 of the federal code that had been passed a few months previously in 1871 that stated unequivocally that no one is above the law, not even a sitting president.
It's still in the congressional record, but in 1874 an unnamed secretary, I believe had Southern Revisionist views, unilaterally revised the law, so it has been improperly copied into the Federal Register ever since.
Police immunity is another interesting angle I hadn't thought of. I was thinking about whether it was relevant to the recent supreme Court ruling, maybe even some ammo for impeachment if they directly contradicted a law with their ruling.