The age gap in Bethlehem kudumba unit is the whole point of the movie and it never felt problematic to me because the film does not portray Ashley as someone who was manipulated or groomed into the relationship.
She makes the choice herself. Justin does not try to impress her or actively pursue her. In fact, he gives her every opportunity to move on, but she chooses to stay.
You may not agree with their choices, but that is what makes the relationship believable to me. The film is about two adults making a complicated choice despite the circumstances.