For me the FL was perfect, she was 19 years old in the first half of the drama, she is like all the teenagers. Both were perfect for each other (in my opinion), she was childish but still intelligent, and he was obviously mature and sad. And together both of them were happy despite of their hard lives.
I feel empty, I saw it like twice but I maybe watch it again. I don't care about the age gap, he is like 900 years old, no one could be old enough for him. Its a fantasy show, don't take it serious