My estimate was being conservative because of two reasons: the world's architecture, clothing, music, hobbies, entertainment, culture, etc is pretty much the same as our world, except with the addition of heroes and villains, which would imply that this setting takes place
20 minutes into the future. If society hasn't apparently changed that much, then I'd be hesitant to assume that the story takes place more than fifty years from the present day. Furthermore, I would not make the mistake of assuming that OFA was only passed on after a full generation. Given the nature of the quirk (can be passed on at will) and the implications it has for its users (OFA users are naturally going to be heroes putting their life on the line for others, risking disability or death), I'm inclined to believe that OFA was passed on several times within a generation(s). Given the nature of their line of work, it'd be imperative for OFA users to find a successor who can inherit the quirk should they fall in battle, and they may decide to pass it on within a fear years of acquiring it due to accumulated injuries, or a feeling that they no longer want to be burdened by its power (but still believe in the need for OFA). Thus, why I extended my estimate to 100 years.
While I believe that AFO is probably very well old thanks to a healing quirk he surely acquired down the line, I seriously doubt he's over a hundred years old. If he was, then the characters would've almost certainly had commented on him or some incredible villain who had endured for a very long time. We know that there was a pseudo dark age before All Might showed up, but AFO isn't mentioned as being a part of it, which leads me to believe that he was either inactive during that time or wasn't present at all. After his brother passed on his quirk, AFO isn't mentioned as having encountered OFA users until Nana Shimura, All Might's direct predecessor. I take this to assume that AFO lost track of OFA (or perhaps didn't even know OFA existed) until he ran into Nana. Furthermore, (episode 49 spoilers)
AFO seems to have embraced the idea that he will be succeeded by Shigaraki and is content with remaining in prison. It came off to me as if he has accepted (or perhaps even wants) his time coming to end, and has been preparing the way for the next generation of villains. He shows no hesitation in staying behind to fight All Might, which implies that he was either going to sacrifice himself to save his pupils, or he was content with dying in battle. Either his healing quirk can't stave off the inevitable forever, or AFO has lived for so long that he has grown tired of the world and wouldn't resist death. See the
"who wants to live forever" cliche for more on that.