Matt Cutts, Google engineer Quote:
"over 200 signals that we use in determining search rankings."
Domain age is just one of them..
I mix and match, if I find a decent aged domain with my keyphrases in it I will buy it and throw up a site on it. If cannot find one with my keywords then I buy new.
You can strike gold by buying an old domain name, with existing backlinks, keyword rich and registering it for 3-10 years. That's the ideal scenario.
Example: I bought the domain franchisesireland.com - it was about to expire and I fully expected the owner to re-reg it at some point which they never did. I waited for the grace period to be up and bought it for £10 despite it being worth closer to £1000 or more. It had PR3 and a tonne of backlinks and was registered first many moon ago. It currently sit's number 3 in Google for the phrases "
franchises ireland" and is an excellent little site that I have done minimal SEO work on and brings in good quality website traffic.
Had I gone and bought the domain franchises-for-sale-ireland.com /.ie /.co.uk /.net etc the same results would not have been achieved without some decent SEO work.
So at the end of the day a good domain with all the benfits can save you time and money.