``... The original temple, named by modern historians and archaeologists Wakakusa-garan (若草伽藍), was lost, probably burned to the ground after being hit by lightning in 670. The temple was reconstructed but slightly reoriented in a northwest position, which is believed to have been completed by around 711. The temple was repaired and reassembled in the early twelfth century, in 1374, and 1603.
...
The five-story pagoda, located in Sai-in area, stands at 32.45 meters in height (122 feet) and is approximately 20X20 in width and is one of the oldest wooden buildings in the world. The wood used in the center pillar of the pagoda is estimated through a dendrochronological analysis to have been felled in 594.''
Depends what you mean by ‘structure’. If you mean extant building then Hōryū-ji[1] has a claim. If you widen your definition then there’s structures like Woodhenge[2].
If you want to be specific, then I mean where wood is kept under significant tension load.
So I guess woodhenge doesn't count since that's compression load. (Only tension load actually stresses the wood fibers in a way that's comparable with the product discussed in the article.)
The problem with the other responses is the original concern was rot. Poorly installed wood will only survive a decade or two at most, often much less. Properly installed wood will never rot. Its not an inherent quality of wood to always rot. The discussion topics are revolving around historical wood structures having failed due to generic building lifetime issues like fire, warfare, mostly economic obsolescence, poor or no maintenance, nothing to do with the material itself.
How old is the oldest wood structure?