
I'm not sure Eorzea would have actual age of consent laws, or if they did I doubt they would be all-encompassing for all cultures and locations in Eorzea (i.e. Seeker tribes). But if I had to guess, I'd say that around 16 is probably considered the age of adulthood (and that sleeping with anyone younger, unless you're around the same age, would be a taboo). As for age gaps, I think large age gaps would pretty normal and not quite as frowned upon as they are in modern society IRL.
In the real world, significant age gaps are often considered weird because someone who's 20 is still typically getting their life together, likely in college and not yet starting a career, etc. while someone in their 40's is expected to to have a stable job, a home, a family, and be firmly cemented into adulthood. However, in a more medieval-ish setting where after your late teens you're an "adult" and in the same place in life as everyone else struggling to get by, just with less experience and wisdom to use... large age gaps aren't quite as strange--it might even make sense to settle down with an older somebody possessing more wisdom, a home, a steady income, etc. than someone your own age in order to have someone to support you as you're just beginning to wander out into the real world.
In the real world, significant age gaps are often considered weird because someone who's 20 is still typically getting their life together, likely in college and not yet starting a career, etc. while someone in their 40's is expected to to have a stable job, a home, a family, and be firmly cemented into adulthood. However, in a more medieval-ish setting where after your late teens you're an "adult" and in the same place in life as everyone else struggling to get by, just with less experience and wisdom to use... large age gaps aren't quite as strange--it might even make sense to settle down with an older somebody possessing more wisdom, a home, a steady income, etc. than someone your own age in order to have someone to support you as you're just beginning to wander out into the real world.