I think this bias stems from the perception that people who are older are more established in life and their ability to support themselves and possibly their family is not dependent on the success or failure of the company.

I think the model that most people, including investors, see when they picture early stage start-ups is a small team that is so connected to the success of the company that they are willing to work themselves to unhealthy lengths if need be to succeed. That image is often associated with young recent grads or drop outs because they have student debts, are less likely to be able to find another job, and don't have any substantial savings to fall back on if the company goes under.

On some level this does make sense, if you are relying on this company succeeding to be able to earn a living then you probably want to be looking at the guy next to you and see a guy in the same boat. However it fails to take into account the experience or expertise that may be brought into the company by someone who is older and has proven the ability to succeed in this industry, which may lead to the success that will earn everyone in the company a living.