How old is all might? Endeavor is currently 46 years old and his eldest revealed kid is 22 which means Endeavor likely started this plan at around 23–24 (I say 23 factoring in his wife’s 9 months of pregnancy).
The video Deku would watch as a kid of All Might’s debut has been described by Deku himself as “old.” He likely made his debut before Deku was born, meaning using Deku’s age to guess All Might’s is futile. I honestly think All Might is in his late 40s to mid-50s because that makes the most sense with the loose timeline we have been given.
Someone talked about Endeavor’s plan to raise a kid to overtake All Might and that’s a good measuring stick.
If his son Toya ends up older than Fuyumi, then Endeavor probably started around 21–22 instead. It hasn’t been revealed who was No. 1 before All Might came on to the scene, but my guess is that it was probably Endeavor or some other hero. This places All Might in his late 30s or early 40s depending on Toshinori’s age when he became a Pro Hero.