If I'm following the timeline right, she's eight about to turn nine and she said something about him being twelve around the time when she was seven. So I want to say an age gap of about 3-4 or 4-5 years.
Thats not much. But why is she so tiny or wait why is he so tall already
I want to say maybe it had to do with the abuse she had been receiving from her mother, or unlike the ML who was training and cultivating a few years before her she falls a little short. There was a scene where the elders and her father were concerned with her being so small so maybe they'll address in the future.
How much are their age gap?