There are two sites that give ready answers to these questions. SEOlogs.com (www.seologs.com) determines the approximate domain age of a site once a URL is submitted to its Domain Age Check Tool. Here's what I gathered: Google's domain was created on September 15, 1997; Yahoo, on January 18, 1995. As of March 31, 2012, Google's approximate age is 14 years and 6 months old. Yahoo's approximate age, on the other hand, is 17 years, 2 months and 11 days old.


According to the same tool, Facebook is older than Friendster by 5 years (see information below). Notice that SEOlogs uses the word 'approximately'. It's because most websites don't immediately buy their own domain names during inception phase. Some webmasters use free webhosting sites before fully launching their own sites to the public. Hence, there's a big chance that most of these sites are older than their actual domain age.
Below are some popular sites and the dates their domain names were registered.
- Youtube.com: domain created on February 15, 2005
- Facebook.com: domain created on March 29, 1997
- Friendster.com: domain created on March 22, 2002
- Twitter.com: domain created on January 21, 2000/Aug. 31, 2011 (please see update below)
Useful sites for bloggers: - Blogger.com: domain created on June 22, 1999
- Blogspot.com: domain created on July 31, 2000
- Wordpress.com: domain created on March 3, 2000
- Wordpress.org: domain created on March 28, 2003
- Alexa.com: domain created on June 17, 1996
- Chitika.com: domain created on January 28, 2001
- Infolinks.com: domain created on July 23, 1997
- Triond.com: domain created on March 14, 2006
- Expertscolumn.com: domain on January 3, 2009
Popular sites in the Philippines - Abs-cbn.com: domain created on January 18, 1996
- Gmanetwork.com: domain created on May 13, 1997
- Philstar.com: domain created on June 26, 1996
- Inquirer.net: domain created on July 29, 1997
There's another site that determines the domain age of a website. It's called URL APPRAISAL (www.urlappraisal.net). If you own a blog, you might find this tool very useful. Not only does it tell you about domain age, but it also gives an estimate of a site's value. As of March 31, 2012, Yahoo's estimated site worth is $41,726.30; Google's, $34,293.54.

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Update:
The two sites give different dates for Twitter's domain age. I'm placing here the screenshots of the results given by the two sites.
Twitter's domain age according to www.urlappraisal.net:


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