Not the worst age difference, but Aang always came off as too much of a kid to me. I could take him seriously whenever he tried to hit on Katara, who sometimes came off as a motherly to him in my opinion.
Which is probs why Kataang was not as popular with 14 year old girls as Zutara was.
Zuko and Katara is exciting to 14 year old girls because that's the kind of man that demographic finds exciting. The tall, dark, dangerous, and handsome brooding Prince you want to try healing. That same demographic doesn't pine after the nice and playful 12 year old boy whose voice has not cracked. It's only when you get older and wiser when you realize that kind of guy would probably make a better husband material than Prince Baggage.
Eitherway Zutara would have been a mess in-universe. The firelord marrying a water tribe peasant girl would have caused a scandal in a lot of circles no matter how you see it. Politically, Mai was always the politically-saavy choice for him. Even if she could have been more exciting as a character.