The age gap between her and Bruce is a little weird though.
I thought about that also, but then I realized it's kind of hard to pin down Selina's age in the show. There could be a wide range there from being still single digit in years to her being in her early teens. I could see them being around the same age. It doesn't help that Selina would be somebody that's already more mature than her age because of the circumstances she lives in making it even harder to figure out.
I don't know if they have ever come up with a set age for the two in the comics. I have always thought they were about he same age or maybe Batman was a few years older. In most modern canon I would have to guess Bruce is in his early to mid 30s with her maybe around there or a little younger. By that the television show is probably off.