Remember that she is also an adult and probably he knows it
She's still a 14yrs old girl who happens to remember her memories in her past life. And I think that is because of her magic. I might be wrong but that's how I interpreted it from Lucas explanation.
She is and have always been a princess. Even is the lived different lives she came back to her true body but experience from her other lives it just that she doesnt remember she was the orinal princess. Lucas knows and explained this last chapter.
LUCAS IS SUCH A..... child. I'm a freakin pedophile, i just can't. :((((