If Deku is born around 2000 (the manga was first published in 2014); his mother (generation 4) would be born around 1975, her mother (generation 3) around 1950, her mother (generation 2) around 1925 and her mother, the first generation, would have been born around 1900. This is assuming each woman has her child at age 25. If, as would be the case with many women in the early XX century, they have the first child in their late teens then Deku's mother could have been born when his grandmother was about 20 which mean his grandmother (gen 3) could have been born in 1955. Her mother could have been about 18 when she was born which would have her (great-grandmother) born around 1937. And the great-great-grandmother, the first generation, could have had her first child at age 15 (not unheard of in 1930s Japan) meaning she was born around 1922.
In any case, quirks would have appeared after 1900 and perhaps as late as the mid 1920s. This would have made WWII and the Cold War very interesting as nations would deploy superhumans as weapons.
Also, I know I'm new, but doesn't this belong in the Fandom section?