We as humans are not bulletproof. With age, some things change. They have to change in order to cope with the changing scenarios and experiences.
I am not trying to say that senior software engineers have some problem or something.
I am just saying that if you are hiring for a start-up (I repeat start-up) then hiring someone older can sometimes backfire.
Obviously, a manager has to manage properly and bla bla but there is an “if” here.
Organically a bunch of junior engineers with a good mentor can have better chemistry than a group of senior engineers.