Sure, you need some senior developers mixed in as well but young energetic devs are more bang for the buck.
From a "getting things done" perspective, I would vehemently disagree with you. If you care about things like 'having more reports' and 'spending investor money', then yes, you're correct.
The correct definition for senior developer vs junior developer is 'delivers company value without handholding' (vs 'with handholding'). If you've lucked into a young, hardworking dev who can do the former, remember to pay them well and don't burn them out. That crop will produce year after year for a while instead of just once or twice.
TBH I've only ever seen this from middle-aged company men.
What I have seen is that when the younger ones look like they're working really hard, it's usually because they don't have a full grasp on whatever the task is, and they're putting in more hours because they keep getting their work wrong.
My experience at Google has been completely the opposite of what you describe. I work with lots of people in their 20s, 30s, and 40s and a fair number of people in their 50s; I'm 35 and I don't feel out of place here at all. I can count on one hand the number of times I've stayed at work past 6 PM; as a parent of multiple school-age kids, there's been lots of flexibility. My team actually has a dedicated video-conference unit specifically to make it possible for those working from home / working remotely to take part in ad-hoc conversations.
The main thing is that you have to be good at what you do. You can't expect to just skate by. I've spent a lot of time mentoring new-grads and those in earlier stages of their career, and this has been recognized and rewarded. I've also written a ton of code and shipped things that worked the first time.
In the Valley / consumer focused apps - they are often focused on both very young target users - AND - very new frameworks and technologies. Plus they work insane hours.
I worked in Software in Telecom as a young man, and I was surrounded by middle aged geniuses, I felt useless.
My buddy worked at a big consulting firm doing .com stuff, he was a primary lead.
So yes, it has a lot do with products, the industry, how fast things change etc..
I enjoy the frothiness of tech, but it's a career danger.
Most of the people I knew working at the telco were in their 60s and started their jobs there out of high school. They were incredible domain experts and had retired from the phone company once already _with pension_. They were all rehired and getting paid huge rates on top of their pensions and some were even going into their second retirement.
Forty plus years at one company and these people were making a fortune and their knowledge is literally irreplaceable. Some even were experts across domains because they were smart enough to just move departments when they got bored instead of entire companies.
Very stressful job though.
But in 20 years from now, we'll still be installing classic land-lines, in which case, they'll still be around.
Idea: build a wireless framework with the robustness of land-line efficiency - good enough for 9/11 calls and emergencies etc..
I've worked with plenty of tech people who just don't grok things as simple as network transmission times or that you can't 'just run a long enough wire (and through any conditions)'. Physics always gets its way. (Side rant: please do your best not to try to send me 20MB of data bounced off of a tin can spinning around the planet, down through dense, negatively-charged clouds to a mixed-elevation terrain through feet of steel and concrete surrounded by 1000 other phones like mine just to view your website/500-word article, plox. kthx)
We're very lucky that our systems are resilient (as in fault-tolerant, not inherently-stable) enough that most of us can work at very high level of abstraction and never dig deeper.
Phone people have to get this or their systems don't work right away. Same with power transmission. We think of these things like they're separate systems and domains from ours, but we're just very far removed from that layer. Everything we build is a client of these networks. There will be bugs or better designs that elude you if you don't understand this.
Just to drive the point home, there's this classic: http://web.mit.edu/jemorris/humor/500-miles
Better to hire the social inept, easily manipulatable young guy, than your future competitor for your very own job.