There are two factors involved here:

1) Because she’s 18, she feels that the age difference is too big a gap and may cause a big problem. If she was 28 and you were 39, I pretty much doubt it would be an issue or if it is, it wouldn’t be such a “big” deal. The last girl I was with was over 10 years younger than me. Not even a mention of the 10-year gap cos' she was in her late 20's.

2) It is how she perceives where this liaison is heading. She probably wants to be in a LTR. Hence, the age gap. And depending on where you want this to go (LTR or a bit of fun), I suggest you be honest with her.

It is better to be honest and risk losing her than beating around the bush and stringing her along cos’ in my experience, if a girl has a bee in her bonnet about the age gap, it causes more hassle that for what it is worth. My two pence…! Good luck fella.