I've been thinking about your question a lot over the past few days. Here is my answer: I think that an age difference *may* matter, but it all depends on the 2 individuals. Regarding the *online* relationship, I don't think it makes a difference if the relationship is online or not... more that it comes down to *maturity* levels of the two. I agree with nightwork that if they are both adults it is OK... but on the other hand, I strongly believe it's important that both parties of a relationship are on similar intellectual maturity and emotional maturity levels. This doesn't necessarily mean they have to be similar in chronological age, just that they should be able to discuss things at similar levels.
Speaking for myself and looking back to when I was in my early 20's, I *know* I wouldn't have been mature enough to have a relationship with an older person. Even if in a situation if I *thought* I was mature enough, hindsight tells me I wouldn't have been. Then again, I wasn't very emotionally mature anyway. From the other side of the fence, Wychic's answer proves it can work out.
So to conclude, I think in some cases it will be OK, in others not. It's hard to judge an individual situation. My feeling is that if both can discuss things at the same intellectual and emtional level that they are normally at (no-one is 'reaching up' or 'stooping down' to the other's level), then it is OK. If people are being something they're not normally, then it's a sign the age gap is a problem.