The Giant Sequoia named Grizzly Giant is, at between probably 1600-2000 years old, the oldest tree in the grove.

How to get to the Grizzly Giant: On the far right of the parking lot, take the right path to cross a pavement road where you will see a fallen sequoia with it's huge wide root base. Walk up the sloop and some stairs, and you will be greeted by this giant sequoia.

Massive and ancient giant sequoias live in three groves in Yosemite National Park. The most easily accessible of these (spring through fall) is the Mariposa Grove near the park's South Entrance, off of the Wawona Road (Highway 41).

Episodic surface fires have swept through giant sequoia groves for many centuries. Nearly all of the largest and
oldest sequoias have huge basal fire scars that bear witness to these ancient flames.

http://www.redwoodsinyosemite.com/sequoias.htm

Just a short drive away is the Mariposa Grove of Giant Sequoias. Giant sequoias are also known as Sierra redwoods - not to be confused with the coast redwood, which are found in a separate range along the California coast. Giant sequoias are native only in isolated groups on the western slope of the central and southern Sierra Nevada. There are three groves of giant sequoias in Yosemite National Park - the Mariposa, Tuolumne, and Merced Groves. Of these, the Mariposa Grove is the largest and most often visited. It contains about 500 mature giant sequoias, and you can spend several hours to a full day exploring this wonderland of towering giants.

Start: Mariposa Grove parking area. (From May through September, it is highly recommended that you take the free shuttle bus.)
Various trails exist throughout the grove. To reach the Grizzly Giant and California Tunnel Tree, follow the trail that begins at the far end of the parking area.

http://oregonstate.edu/trees/con/giseqgen.html

Giant sequoias are the largest trees ever to inhabit the earth, and are among the oldest. Heights of 300 feet and diameters of 30 feet are not uncommon. Their ages commonly range from 2,000 to 3,000 years (only bristlecone pines are older). Although once widespread, giant sequoias now occur only in the Sierra Nevada Mountains of central California. Since 1890, giant sequoias have been protected in Yosemite, Kings Canyon, and Sequoia National Parks, as well as in smaller individual groves. There is only one species within this genus, Sequoiadendron giganteum; it has the same common name as its genus, giant sequoia.
 

Directions: I used 41 to get to the Mariposa Grove on this October 2006 trip.