OK, does anyone consider why there is an age limit? Because minors are often immature and have bad judgement because they are young and lack experience. That does not mean all minors are immature, that does not mean all adults are mature and have good judgement, but overall the younger you are the more likely you will be immature and unable to make good decisions. Thus the reason why minors are given extra protection by the law.

So if Mr. OP is destined to be the soulmate of this wonderful 15 year old he will be able to wait the three years until she is legal and then they can go off and do all the wonderful adult things we adults do. If he can't manage that, then he should break it off, instantly. He should have respect for both her, and acknowledge that she is still a child, and the law.