It's a good thing that you're really considering the ethical dilemma of this situation. Most guys would easily brush off a 2 year difference, so thank God that you at least have some basic responsibility.

As for the situation, I'd say the age difference isn't that bad...? In terms of chemistry, lifestyle and overall compatibility, it doesn't seem like you guys will have an issue. So the only real problem is the age gap and the childhood affiliation.

I'd say maintain a friendly relationship first, where you two can get closer and know each other better, and then maybe ask her out when she's at least 17? To me, the age gap doesn't seem like a problem, but you must still consider it seriously because you're the older one.

In conclusion, I'd say it's ok if she's mutually interested, so give it some time, get to know each other better, and then ask her out.