Others joked that how can I pay back the old cow who eats young grass? Where's your girlfriend? How to reply?

You could say that. You said that as long as you really love each other, there is no such thing as an old cow eating young grass and then an old cow eating young grass. This is also a compliment, which means that your girlfriend is younger than you, and then she is particularly beautiful. Then you are a particularly capable person. Just answer casually. Anyway, it is a particularly superior thing. Why bother about it? If your girlfriend asks, it means that you two are true love, and nothing old cows eat young grass.