
Rather than mentally, it's more like intellectually. He has the knowledge of a 17-year-old's brain. But since his brain is currently developing, his brain is censoring things that might seems to advanced for his developing brain. Which is good since he can grow up as a normal kid and not as an OP too-matured reincarnator.
Think of it like watching the same movie as a kid, as a teenager, and as an adult. Each time, you would have more opinions about the movie, and loon at them differently too.

He got mental regression when he possessed Fran, so he was 17 but after he possessed Fran who is 6yo, his mental got regressed into Fran's 6yo too, its also a possibility that Fran's real personality affected him so he looks more childish than your usual regressor MC who unrealistically became mature suddenly but its better that way since its cute

All he did was remember the memories from his past life. So it didn't really change anything mentally and intellectually to his present self. However, he became more aware and understanding of his surrounding. I actually like this better, compared to the other manhwa/manga wherein they instantly have a personality switch. I like how Fran is just gradually changing, slowly maturing, befitting his age.
Is Fran really mentally 17?
I aren't complaining. It is really cute. But it would be better if Fran was actually 6/7 when he died in his world. That would be a little bit real. Whatever I don’t know.
But I am enjoying for sure.