Let me be clear. I know for a fact that older domains are way easier to rank. This was a simple example, I have many more cases where this has happened.

Google gives a good trustrank in old domains. That makes red flags fewer, rankings higher and pr higher. I assume, that it affects all aspects of the domain ranking, including the Adwords Quality Score.

(Of course, we are talking about a case where the only difference is the domain age.)

So yes, I believe domain age affects QS.

But, personally, I would not send the ppc traffic to the old domain. I would seo the old domain, and send ppc traffic for lead generation to a thin newer optimized domain. Longterm seo for the old domain is priceless.