Adding a weird age gap for "realism" doesn't make it justified, especially since the game isn't entirely historically accurate.
"Applying today's standards, laws, and values on a historical time period is not constructive, although, unfortunately, is done far too often nowadays." --> They're fictional characters. Furthermore, by that logic, you're saying that it's okay people had slaves because it was considered okay back then.
Since it was normalized back then, it makes sense that people had slaves and doesn't necessarily make them bad people, but it was still wrong and horrible and shouldn't have happened.
It's the same with this type of age gap, obviously to a far lesser degree. Normal for the time period, still not cool.