I think it's more an initial aversion to a lot of risk and lack of direction, as opposed to a lack of interest in startups or small companies. At my previous startup and at my current company, we have a lot of 24-29+ year olds (as opposed to 18-23 year olds) joining us. Upon graduation, they likely have some student debt, often don't know what direction to take at 21 or 22 and end up working at a large tech company or for a bank, etc. for a few years (who offers them grandiose promises of big paychecks and stability) before they take the plunge and join a startup.