As a early-twenty-something I only know a relatively small number of engineers 30+, and have only worked with a handful. Compare that to the dozens of engineers my own old that I know, and have worked on projects together. If I founded a company tomorrow I know which 4 friends from college I would want on my team, they're all other twenty-somethings.
The only engineers over 25 I have worked with (excluding open source projects) were my bosses at previous companies. I simply wouldn't know who to hire. Anyway, would any self-respecting engineer take a job at a company found by one of their interns or recent graduates from a few years back? I highly doubt it.
In my hypothetical would I really hire someone who was twice my age? Probably not. To be honest I'd be afraid of their experience. I'd feel maybe their my training wheels. They have a lot more experience than me, will their experience take over my company's vision. Part of the mentality as a twenty-something founder is proving yourself, be that to your colleagues, your peers, your parents, or whoever said you just wouldn't make it.
I find it hard to find other twenty-somethings hiring many people twice their age in the early days. Maybe without even realising it a culture similar to that of college creeps in. Every new hire creates culture, and from my hypothetical I don't have a very diverse team to start with if I start with college friends and other twenty-somethings.
Somewhere in the early days no one seems to be spotting the 'culture' problem start-ups are creating. Was this problem unintentionally created? Or was this problem created sub-consciously created intentionally? Would I as a twenty-something founder sub-conciously create a company where my Dad or my even my cool Uncle wouldn't want to work at? Probably.
I demand a very high premium for the work I do for others, and if I were going to start my own business, I wouldn't hire inexperienced people to do it with. I know the people I need to know to get financing/sales/engineering/etc. and they all have proven successful track records. At best you could bring passion and new ideas, at worst your college culture would creep in.
Don't get me wrong. I enjoy working in environments with younger developers. Mentoring new developers is one of the most rewarding things I do. But my employers have me do it because hiring young folks is a cost of doing business.
That said, I don't think anybody is worried about ageism creeping into 3 person ramen startups. The issue is 2 fold A) if your startup becomes successful enough to need my expertise, and you don't hire me because you don't see the value of experience, that is bad for both of us. B) There currently seems to be severe bias in SV investors against older founders and/or startups that employ early stage experienced folks. Paul Graham has this bias and is proud of it. That is the ageism that gets me upset.
Finally, I will say, writing good software is a craft. Like all crafts, learning from experienced craftsmen is central to getting good at it. If writing good software is something that is important to you (it needn't be, and might even be detrimental) you need to go meet and work with older developers.