Furthermore start-ups also implicitly prefer "just good enough" in order to get your company up and flying enough to quickly accrue investor interest, whereas stability forms the bottom-line of our elder engineers -- and stable systems require far more time than some investors are comfortable with.
Even worse, there is the management aspect. Older people have very different management styles, and throwing caution to the wind is definitely not among their managerial toolset. If your company has older people in positions of power then it definitely scares vast swaths of people away -- from eager investors looking to turn a quick buck and energetic young engineers looking to for autonomy to work without anyone second-guessing their decisions.
Finally, the saying "old is gold" applies quite well, however that gold needs regular polish to remain shining.