I don't want to re-read this, but did I miss something that indicated he had ANY possible romantic interest in this person? It seemed pretty casual except for her "OMG, MC MUST BE LOVED" syndrome. They have no chemistry together...
Luck is a magic fanatic who likes researching ancient magic. She was one of the few people he knew who has also researched ancient magic.
If nothing else, he respected her talents in magic stuff as he himself is a magic fanatic.
So, chemistry wise it's there, but I think it's weird for his friends to portray that as romantic interest.
Filly is in her early 20s no? And Luck's true age is in his 30s? While it is far apart it isn't exactly socially unacceptable levels either.
Luck is 39 by ch 1, probably 40 now. He is 4 years younger than Golan, the guild master, as shown in ch 18.
Filly was a child in pg 17 (maybe around 7?) reading a book that was only first published less than 10 years ago. She is probably 17 (or 19 at the most).
I mean he's at most 32 mentally cause he was basically unconscious most of the time he was fighting as far as I remember because he was using marionette to keep going until he reached the majin lord
He said he used self-marionette when he was sleepy. I doubt he was "unconscious most of the time" given how he was fighting a whole army. He probably need to use Life Drain often to get energy, and I doubt it's something that can be done while unconscious.
Even then, I don't know if that qualify as reducing his mental age. It's no different from people who sleep regularly in 10 years.