Honestly, it depends. I've written in the past on marketing and the generation gap. What I noted in those previous posts is that young people tend to form their identities around what they consume (i.e. types of music, clothes, etc.) and older people tend to form their identities around what they produce (job title, parental status, etc.). It's these competing outlooks that form the generation gap.
So given that, it makes sense that in consumer facing areas where the audience demographic tends to skew younger, younger entrepreneurs are more strategically positioned. Sorry, but no forty year old is going to come up with MySpace or Facebook. Those are sites designed for teens/ twenty-somethings, marketed to teens/ twenty-somethings and used by teens/twenty-somethings. This audience have an innate knowledge of the nuances of the product that no 40 year old, regardless of how much market research they undertake, could understand.
There's an old piece of advice that's often given to authors: write what you know. In other words, don't try to make stuff up, leverage your experience to create. I think that advice applies to the "old" entrepreneur as well. Don't try to design something for the younger crowd. There's a generation gap that you can't overcome. And don't try to design a great consumer destination for your fellow forty-somethings - for the most part, your peers are not passionate consumers (especially on the internet), rather they're passionate producers.
So is there room for great forty something entrepreneurs. Absolutely. Just remember, certain areas are rightly defined as young man's sport.