WHY IS SHE TOO OLD TO BE HIS KID???? That makes no sense to me. First, if he got a woman pregnant at 18, and he's 38, how old would his daughter be? 20. What if he got a girl pregnant who was 18, but he was 16? His daughter would be 22. What if he's 40 (which is my presumption) and he got a girl pregnant at 16? His daughter would be 24 - and the perfect age of a graduate student.
All that being said, I agree Marvel could also have sent him at 25 or 30 (after becoming the BK) on a time-travel adventure (he's been on a few that we know of), and it could have ended with him falling in love in the 80s before being sucked back to the modern day, making Jacks even older than 24.
All of that clearly fits with what we've learned about her and about him. She's his daughter and the woman that he cried about was her mother. The story about conception is just left to be told. I, for one, can see why Dane needs to have family around, and an intelligent, adventurous daughter who later will take up the Black Knight mantle is a great way to fulfill that need!