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Personally I have a lot more respect for a site that's been around for ten years than a site that's only three months old. The older site has proven that it has staying power, and its longevity indicates that its owner values it and has probably put a lot of time and effort into it.
Six months is a good general estimate, I'd say. It's most important at start-up to focus on building the business these days, rather trying trying "too hard" to gain rankings. The algorithm tends to reward solid businesses.
I don't think domain age has much of any effect whatsoever on whether a site ranks. It's just one (very small) piece of the puzzle. How long it's registered for might be a small indicator of trust, but again, very small.
Why the Google Sandbox Effect exists
Put simply, the Google sandbox punishes new sites as a way of combating black hat SEO practices. A lot of black hat SEO techniques involve building hundreds or even thousands of sites overnight through an automated process to build large volumes of backlinks to the sites for which they want to rank. If their first process (say, a link pyramid) didn’t work, they then reconfigure those thousands of fake sites in a different way in an attempt to get more links.
The Google sandbox makes those black hat practices much harder in two main ways:
By punishing new sites by placing them in the Google Sandbox, the black hat SEOs have to wait months before they find out if the scheme of the day actually works. If it doesn’t it will take them months more to experiment with another loophole idea. This also makes it impossible to launch a brand new site with a thousand low-quality backlinks overnight and start outranking legitimate sites.
By creating this waiting period, Google buys times to algorithmically detect and discount these various link-building schemes before they actually do damage to search results. So the black hat SEO builds a thousand fake sites in a link building configuration, but has to sit in the Google Sandbox for a few months first. During that time Google figures out what the SEO is doing and adjusts its algorithm to ignore the benefit of all those links.
[edited by: plc90210 at 6:11 pm (utc) on Apr 12, 2013]
How long someone registers a domain for isn't something that search engines can detect for every site.
Put simply, the Google sandbox punishes new sites as a way of combating black hat SEO practices.
 - actually it is pretty simple, lol - I used to be a domainer...
Domain age having an effect on rankings makes a lot more sense to me. I found an article online that I think does a great job of explaining why the age of a website is a ranking factor
I've never seen newly registered churn and burn sites ranking for competitive keywords. Mind giving any examples?
plc90210, correlation does not equal causation. Domain age sometimes does seem to correlate with rankings, but if domain age is a factor at all my personal experience suggests it's not a significant one at all.
As I said, it only applies to keywords w/ high search volume and even then, the new site can still rank, but just not as highly as an older site would (all else being equal).
How do you determine all else is equal without knowing precisely every single thing the algo weighs and how much weight it assigns each factor?
Example: I started a site in early January targeting two keywords - one with 5,400 exact match searches/month and the other with 33,100 exact matches searches/month. Both keywords are very closely related (they're basically synonyms) and have pretty much the exact same sites ranking in their Top 20 and as a result, both keywords have pretty much the same difficulty level. The only real difference is that one has a higher seach volume than the other. Today, my site is ranking #6 for the keyword with 5,400 search volume and #12 for the keyword with 33,100 search volume. In terms of on-page SEO, the site is pretty much equally optimized for both keywords and as I said before, both keywords have pretty much the exact same SERPs since they are basically synonyms.
Pretty good, yes, but unless they have precisely the same link profile, it's not 100%.
As Robert mentioned, aging of backlinks may be another factor, and one that would tend to coincide with domain age (in a big enough sample, at least), making it hard to tell which factor is the influential one.
IMhO, I think the length of continuous time a site's content/purpose has been live has more to do with just the age of the domain. Meaning, if a site is a blue widget site for 3 continuous years and the domain is 3 years old, I think that has more value than a 10 year-old site that has only been a blue widget site for 1 year. All other factors being equal.
Clear as mud?