Re: Is it considered rape?
If he had to be 18 for it to be considered rape, then when she was 14/15 and he was 18/19 then it was rape.
Despite the legal issues, when you are that young, as you said you do not understand the consequences of engaging in such activity, while someone who is 16 has more of an idea (though still perhaps not fully).
Whether or not you agreed does not matter, he should never have put such demands on you. In many ways you were still a child.