A large amount of stress is amplified by mini-dramas that could have been avoided entirely by people with more life experience and fewer human cogs in your processes usually means less mess to cleanup, allowing you to focus more on what's important.
Junior folks bring their own energy, flexibility, hunger for learning and the resilience to unclear future. As folks get more mature, growing egos, stability expectations, tendency to settle down can get in the way.
Overall, this boils down to the fact that teams and people are not just head counts and there's a factor of chemistry need to be considered depending on the culture, founders, geography and the business itself.
... arrogance, hunger for prestige and one-upmanship which leads to fixation on irrelevant outcomes, laziness in learning "old things", and rejection of instructions.
But old folks can also do these things.
And yeah, the older I get, the more I appreciate older people; I prefer no-nonsense 35+.
Scaling from a handful of people, to more than a dozen creates a significant jump in stress, and dilutes focus, for a non-commensurate jump in productivity. Any pre-PMF startup should be as small as humanly possible.
I was / am by no means the best at my job technically, in fact some of those new guys were better out of the gate technically than I was in some ways. But I could get along with the new guys and enjoyed working with them. Over lunches and small talk I would quietly have discussions about "how to pick your battles" / "yeah that's absurd but here's an easy way to get around it rather than fight it" and more generally "amazingly easy things you can do to make your boss happy that cost you nothing at all". These lessons of course weren't me just telling them what to do, just planting ideas that they eventually would discover, hopefully sooner rather than later.
They were basically all the things I learned the hard way ;)
Hopefully you haven't just stored it all in your brain :-)
Those problems don't get solved by excluding that group; you have to include them or they don't learn. A good amount of maturing comes from being near folks wiser than yourself.
To put it another way: the higher your ARR, the more responsibility you have for others. The first period of a start-up is the riskiest and working with fewer young people significantly reduces that risk, especially in today business environment.