A quick search finds the oldest pyramid in Giza was built about 2700 BC. They know the dates of those pretty well since there was a lot of writing left. There are older Egyptian pyramids but they're not in Giza.
I don't know how accurate the dating is of the Methuselah tree. With normal trees a forester can use an increment borer to take a core sample from which to count the rings. It takes some skill to be able to hit the center of the tree on a well formed tree. On one that's gnarled and has missing parts like many of the old bristlecones, it might be pretty tough.
The definitive way on a hard to count tree is to cut the tree down and count the rings. Obviously you don't want to do that to what's possibly the oldest tree. Unfortunately someone did in the '60s- a researcher cut down a bristlecone that turned out to probably have been the oldest one (that we know about). It was an honest mistake but it kind of tanked his career and greatly affected his life. There's an article about him out there somewhere.
I've hiked the trail that the Methuselah tree is on. They don't have a sign on it for obvious reasons. The whole area is worth traveling to and the hike was pretty good too. The forest service station that was there had a bunch of cool displays and sections of trees to view but it burned down a while back.