Co-founder/C_O types in their early 20s are often intimidated by people 35 and older, especially when those people have a long record of working in tech and all the war stories to prove it. The thought of hiring such a person, even when they are superbly qualified for the job and amenable to the compensation package, makes young entrepreneurs a bit uncomfortable.
I think there are two specific fears that drive this feeling of discomfort. First is the fear that older employees might see right through the fact that they have no idea what they're doing and call them out on it.
Second is the fear that older people will destroy the culture through their relative lack of enthusiasm and exuberance. In young founders' minds, it's better to hire someone who's been drinking legally for all of 18 months and who can be counted on to nod earnestly when you talk about "our mission" at the all-hands.