Isn't Jaqen too old for Arya ? Gendry is closer in age to her
Bookwise Jaqen is 18 years old. The age gap between Jaqen, Arya and Gendry is the same as between Rhaegar, Lyanna and Robert. In Westeros “a man grown” is a boy as young as 16 years old, that’s why Gendry is often described as “almost a man” or “the closest thing to a man.” Jaqen refers to himself as “a man” like Loras does but Jaime thinks Loras is still a boy. The same goes for Jon, Sam and Pate. Technically they all are “the men grown” but they are still teenagers. GRRM gives away Pate’s age in A Feast for Crows to let us know how old Jaqen is.
In the show we are talking about two grown up actors performing their roles not actual people starting a relationship. Usually actors portray characters younger than themselves. In many high school themed shows/movies the main characters are played by the actors well in their twenties. We can assume that in the show Jaqen is a seasoned assassin who may fall for a young but adult apprentice. Arya is a grown up girl in the show and so is Maisie. I don’t see anything wrong about it as long as the relationship is consensual and people involved are adults.
Thank you for the question :)