The answer depends on the species. Conifer lifespan at the top end gets measured in thousands of years, and a giant sequoia can live for more than three millennia. The National Park Service records a maximum of 3,400 years for one. Coast redwood sits a tier below that, with the oldest of them reaching 2,000 years.
How old redwoods get is the part most people underestimate. 600 years is a routine age for a coast redwood rather than a record, and the oldest trees run more than three times longer than that. A tree sitting at that routine age was already full grown centuries before the United States existed. That is the ordinary case, not the outlier.
The two figures do different jobs, so check which one you have been handed. A maximum describes the few trees that got every condition right and were never knocked down. A routine age describes what the species manages on a site that suits it. For coast redwood those numbers sit 1,400 years apart, and you will see both quoted as the conifer lifespan for the same species.
Age by itself does not end these trees. For mature coast redwoods, the most frequent cause of death outside of logging is windthrow. The wind takes the whole tree over instead of killing it where it stands. A redwood can hold its ground through centuries of storms and then go down in a single night, and the count of rings stops there for a reason that has nothing to do with being old. If you want a working answer on any conifer lifespan, that is it.
Giant sequoia age tops out at 3,400 years, and the pressure on those trees shows up in a different form. During and after the hotter drought that ran from 2012 to 2016, 33 giant sequoias died standing. Native bark beetles and fire damage at the base were both linked to the losses. Two pressures reached the same trees at the same time.
A conifer that dies on its feet is reporting something other than wind. In the sequoias lost that way, beetle activity and fire injury low on the trunk showed up together.
Living on that scale also changes the math on reproduction. Across a lifespan of several thousand years, a giant sequoia needs just one offspring to reach maturity to keep the species even. Every other seed and seedling it makes can fail. That single success is the whole requirement, and the tree has millennia to land it. You would call that a slow bet anywhere else.
Long as these numbers run, giant sequoias are not the world's oldest trees. Other species live longer than they do. No verified age for those species could be checked for this answer, so I am not going to attach a figure to the claim. Treat any oldest tree number you run into with the same care, and hold off until you can trace it back to whoever measured the tree.
None of this tells you much about the conifer in your own yard, and that is the honest part. What ended the trees in this answer was wind on a mature redwood, and beetles with fire damage low on the trunk in the sequoias that died standing. Neither figure was measured on a garden tree, and no source here compares one kind of care against another. For the species growing where you live, a regional extension service or a certified arborist is the place to take the question.
Read the full article: Conifers: Types, Traits, and Hard Numbers
