Gonna try to give a simple break down. Warning: spoilers.
ogFL would be considered the same age as ogML. ogML was adopted by the grandpa put of guilt over ogFL's mom abandoning her. He mother requested a lump sum to basically leave, never return, and would use it to raise the baby. Instead, she took the money and abandoned the baby. Not knowing which child she abandoned, ogML was adopted thinking that was the abandoned child. Neither of their actual birthdays are known, so they have the "same birthday" because they were abandoned on the same day in the orphanage.
Yeryeon (FL) is younger, since she was born from the CEO and his wife after they married.
No idea how old Eunwoo (ML) is in relation to everyone, I don't think it comes up, but he should be similar age to ogML.
So it seems like the fl’s father got drugged before his engagement, so does that mean the original fl is older than both the fl and her brother? And when was her brother adopted?