What do you think when dating a younger guy?

I'm (16m) and I'm about to get in a romantic relationship with a friend (19f) and I'm slightly worried as she will actually be turning 20 before I turn 17, it's scares me because I feel like she will be wanting to get on with her life at that point and I will kind of be left behind. But that's how I see it, I'm wondering if age is really that much of a factor and if it is, is there anything I need to know about being with an older woman, both romantically and sexually, as this will be my second relationship I'm quite... Inexperienced