Speculating I would say:
- chose younger hire because cheaper and can be moulded into company specific role. - younger candidate more easily managed by less experienced (cheaper) manager. - wants to keep culture "pure," and needs kids to drink kool-aid. - values power and control over less experienced technically acceptable candidates. - get extra effort and all nighters out of people who think they need "experience." - want to leverage kids love of novelty to react and respond to developments that seem like minor details to people with experience. - younger people have less sensitivity to change. - early stage companies want to reduce exposure to risk from having to re-negotiate once key developer has them by balls.
If you are old, present as harmless. The more hippy dippy and spergy you come off as, the less threatening you will be, the more you will disarm clients/employers.