It's more like a 6-5 year difference. At the start she was 11, he was 17, she also turned 12 like 6 months into the story timeline.
They also don't know exactly when her birthday was too because of her garbage mom.
So the age difference is acceptable in my opinion. IRL my boyfriend is 7 years older than me.
I hear you, and I agree like 50% that her being 16 is what makes it weird.
But she's matured a ton in the recent chapters, or supposed to have because everyone keeps mentioning it. And Emel is honestly pretty emotionally immature, the way he handles and deals with his emotions. So in that regard, I think she's more mature than he is in some ways.

So...how big was their age difference again?