Yeah luffy might be mentally 9 years old but boa has the mentality of a 9 year old in someways too most 9 year olds these days think they are inlove with someone they only just met just like boa hancock
Luffy is mentally 9 but remember, Garp is his granpa. Luffy inherited Garp's family jewels which means his mental age doesn't matter...He'll always be PK level down there.
Wasn't there just a chapter showing the age of Kyros and Scarlet and there was like an 8 or 9 year gap in their ages?
Not to mention I'm assuming there was an age gap between Roger and Rouge since Roger I assume was around 45 - 50 at the time of his death.
The way I see it it's One Piece anything can happen besides it's not about romance Luffy will be probably do her once leave a child and run off somewhere.