* Salary. Older employees are more experienced, but, just as importantly, they've already job-hopped a bit, so their base is a lot higher. And they've probably a lot more personal financial responsibilities. All told, they're unlikely to go for the same salary as a new grad.
* Interview process. Anecdotal experience, but, the interview process is structured towards new grads. They've studied algorithms recently, so it's at their fingertips. Unless you've brushed up before interviewing you can look sloppy and unsure in comparison. This can be harder if you've an existing job and family commitments. (Note: I'm not saying you shouldn't study; just pointing out that this can happen.)
* It's hard to gauge experience. Simply because a person has been in the industry for 10 years doesn't mean that they're "more experienced". It's very tricky to gauge: it's easy to tell when someone is less experienced than you, but much harder if they're more experienced. Plus, how do we evaluate experience? Design questions? Software architecture? Looking at code they've written? It's very fuzzy, and a lot depends on the skill of the interviewer and their own experience.
* Finally, we devalue experience. A personal bugbear is hearing hiring managers say that they can train up a new grad to be a tech lead in about a year. Unless you've worked on a couple of largish projects - and seen your own designs evolve in response to requirements, I question that strategy. But, who knows - maybe I'm completely off-base there.
EDIT: I'd like to raise a final point: sometimes the impact of experience is only apparent when you've to scale your product architecture or your team. Assuming that most startups fail (i.e. never make it that far), this means most very early stage teams can get by with an ad-hoc process and/or system until they pass that hump.