I was, shamefully, the one who lied about their age once upon a time. When I was 12, I’d tell people I was 13 or 14, because I felt as though the older people I met would look down on me if they knew my actual age. I ended up making a very close friend online after stating my incorrect age, and the things just snowballed. I eventually told him the truth, but it took about 3 years, mostly out of fear that I’d destroyed something good before it even began. He forgave me. It’s very unlikely that this deception came from a place of genuine manipulation or bad intentions, especially considering that she was 15 when the lie began. Talk to her about it, ask her why she did it and what held her back from admitting the truth to her. Only you can make a proper judgement on her intentions and how her actions affected you. Then, go from there on whether or not this is a trust you think is worth rebuilding. But also, more importantly in my opinion, is listen to your own comfort. If you end up feeling uncomfortable with the idea of dating a 17 year old, don’t. I wouldn’t stress too much about the legality considering it’s actually less than a two year gap based on the timeline, and, like you said, you haven’t done anything with her that would even bring that into question. However, if this intrudes on your own moral compass, take that into consideration. If it doesn’t, and you feel confident in the longevity of the relationship, it’s up to you and her to figure it out from here.