Well it is okay, of course, it's not a 10 years age gap, that would be a problem. It is just recommended to have someone your age because both people are going trough the same phase and want similar things. 3 years is not that much of an age gap, especially if she already hit puberty that would be ok.
The reason they say just that is because it's not a problem. That doesn't mean they don't care or that they don't know better or that they don't respect it. It's not just an adult, it's a handful, If a lot of people say that, then it's more likely to be true. So don't sweat it. A 16 and a 8 yo kid thats a problem.
If you don't like the response you get and you think you know better then good for you.
Asking such things on this category is not really going to help because I think a lot of people here will probably hurry up to judge a situation without taking everything in consideration. They might not even really know what pedophilia is.