While I do agree in theory that eight years isn't really that much, for people as young as we are (I'm 24) eight years is a big difference, psychologically.
For one thing, someone who is 19 has a vastly different outlook on life than someone who is 27. Most (because really, what we are talking about here is most, not all) people who are 27 are somewhat ready to settle down in some aspect or aspects. This isn't necessarily relationship-wise, although that is certainly one aspect. For an example of another aspect, you may be willing to settle economically, deciding on what career you want to persue for the rest of your life.
That example really highlights the difference between the two ages. Someone who is 19 usually doesn't look to the future, but is more steeped in the present. People who are 19 aren't thinking about any sort of settling, any sort of permanence. It's an age when people generally are very unsettled - it's when most of us experiment to a great degree.
It's also why the difference between 30 and 38 is much different than 19 and 27. The 30/38-year old couple are both in a stage of life where they are both more steeped in permanence than change. Of course, they'll have their own problems once the 38-year old hits the mid-life crisis.
I would be extremely wary of going into this thinking that it is going to be a long-term relationship. On the other hand, if you're just looking to have a little fun, hell, have a little fun. What ultimately counts in the end is if the time you spend with her (or anyone) makes you happy, whether things work out in the end or not.