You say it isn't a big deal, but I don't think that's necessarily true! Having sex for the first time may not be a big deal to everyone but it certainly
can be, and if it is for you, it's important to acknowledge and respect that. What I'm hearing is that you don't feel ready for sex with her just yet, and if that's how you're feeling, then I certainly think holding off on sex for the time being is going to be the best thing for you. Sex isn't something to rush into or convince yourself to have when you aren't up for it, for whatever reason.
Do you have a sense of why you're scared to tell her you aren't ready? That's what I'd recommend as a first step, so if you want to talk about how to have that conversation, that's something we'd be happy to do.
It sounds like you may have some additional reservations about the age difference, and if you'd like to discuss any of those we can certainly do that as well.