When to take the next step down the dating avenue?

I met a girl on an online dating sight. Our initial contact with conversation was the 3rd week in July. We sent a few emails out to each other, that had a lot of depth in them. After about a week and a half, we had our first date. We are now up to date #5. She is about 5 yrs younger than me, but that doesn't bother me at all. I guess since I've been out of the dating scene for a while, when do you know the right time to move forward? I mean, is there a general rule people have in their minds that after X number of dates, moving on to the next step? I guess you would characterize us as dating, but when is the time to say "Hey, I like you, and just interested in dating you and seeing where this will go?" Kind of like an exclusive thing. Maybe it sounds cheesy, but there is always a fear that if you don't take the reigns, that maybe they will loose interest, or see that I am not interested because I am not moving forward in the process.

I hope this makes sense.
Appreciate any feedback.

Are you wanting to take the next step because you have strong feelings for her?

Yes. The feelings continue to grow the more I talk to her, see her, and with our dates. We've done a wide range of things in a short time (Trivia night at a bar, nice dinner/jazz bar, made dinner together, etc).
I've gone on a few dates with women from the online world. It always seems like after a few dates, I was the one always putting effort in to get anything going, and the dates finally just sizzled out. But with this girl, there is a lot of happiness, pro-activeness, and just over a great person that I met.

There isn't a standard timetable (that I know of), but if the feeling is right for you, then now is the time. Talk about how much you've enjoyed being around her. Let her know that you're not seeing anybody else and hopefully she will say the same.

I met my girlfriend under similar circumstances, and was starting to feel the same way after a few weeks. Our next date was especially great, so I surprised her with a nice kiss when I dropped her off at her place. She was surprised but happy, and we were definitely a couple after that. We've been together for seven years now.