I agree with Jes. This 21 year old based on what you wrote, is just that, a 21 year old that wants an older woman and is putting you down with words and being demanding much like a little kid right?
That's what you have there, a little kid. You know full well that won't work in the real world and if you went further, what will this kid do then? Leave you alone? I have a feeling not.
If you wanted my thoughts, I'd say 1) you have both been together from a young age off course you are best friends. 2) if you don't work on a relationship and keep spontaneity into it, fun, laughter, passion it turns to just "best friends" and it is hard to change that.
Often I think if one person doesn't try, it's because they don't know how. Or, they are fully aware that the spark has gone and worries about the outcome if they do try.
You don't have children. Go on a holiday together, spend a week away together talk about past things that you both remember that "are" your good memories, laugh, touch, hold hands and see how it pans out by the end of it and if it works, keep it alive after, don't allow work and life to get you back into a rut.