My thought was that startups target younger devs as they believe that they'll be more 'liberal' engineers. I think the graph shows that to be false.
edit: well I don't think the graph shows anything - but it suggests that age and liberalism are not correlated. Many people have suggested that the survey is flawed - and that Yegge's original post is biased against conservatives.
I always assumed startups target younger devs because they generally fit startup culture more: cheap, work horses, full of energy, willing to take on risk.
When you've got a family you're more likely to want real cash instead of equity. Many startups are cash poor and they wish they could get some geniuses who'd live on ramen noodles for two years in hope of a future payout. A 40 year old programmer who's good can get a job with good salary and benefits and doesn't want to hear about it.
I think many IT employers like younger workers because they're easier to intimidate. If you've lived through a few "charge of the light brigade" projects, you eventually learn how to say "no".
Please, now that you have a half-billion records in Mongo, I want a report on statistics of these in ways you hadn't thought of before on my desk tomorrow...