I've been wondering, how long has President Snow been president when the "Hunger Games" books (or movies) take place? Given the described medical possibilities, a 75-year presidency doesn't seem unreasonable, but is it explicitly mentioned anywhere?
I don't recall Collins ever stating it in interviews, but it must have been at least 25 years since President Snow had Haymitch's mother, younger brother, and girlfriend killed after the Second Quarter Quell, which was the 50th Hunger Games.
As you noted, the medical technology in The Hunger Games is advanced but the Hunger Games wiki claims President Snow is 76 years old, which would imply to me that there was at least one other ruler of Panem after the 1st rebellion.
Snow had been President of Panem for at LEAST 25 years, as it was mentioned that he was presiding over the nation when Haymitch won the 50th Hunger Games (the trilogy takes place with the 74th and 75th Hunger Games, before the second rebellion and the overthrow of the Capitol). It is HIGHLY doubtful (as some have suggested) that President Snow was leading Panem for the past 70 + years leading up to the Hunger Games Trilogy. The Hunger Games Wikia mentions President Snow being in his mid-70s, age-wise, so he had to have at LEAST one predecessor. That's all we know, as the details of Panem's history is largely unavailable.