About 4.55 billion years plus or minus 1%. That also goes for the whole solar system. The age of the Earth can't be detected by materials found there, the oldest rock that's been found is about 3.8 billion years old.

"While these values do not compute an age for the Earth, they do establish a lower limit (the Earth must be at least as old as any formation on it). This lower limit is at least concordant with the independently derived figure of 4.55 billion years for the Earth's actual age."--The Age of the Earth, Chris Stassen.
Source(s):
http://www.talkorigins.org/faqs/faq-age-of-earth.html

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Or (not to be controversial, simply offering another method of calculation) there are some who have strong reasons to believe it is also less than 7000 years old.

It depends on which theory you choose to base your number on. If you're not interested in comparative theories, ignore the link below.

(I don't endorse the site, but it has a collection of links that might provide some food for thought.)
Source(s):
http://www.earthage.org/EarthOldorYoung/The%20Age%20of%20the%20Earth%20Link...

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