You haven't experienced a whole lifetime. Every moment up until the current [whatever the smallest unit of time really physically possible is] is just a memory of an experience. Subjectively from your perspective there's absolutely no difference between having actually experienced all that and merely having memories of having experienced all of it. You could materialize into existence for exactly one moment and believe you've lived an entire lifetime, but in reality you're just a brain floating in space experiencing one brief moment of a lie told by the chance arrangement of the molecules that make up your being.
Now that's an existential crisis right there. There's no way to determine if I've literally flashed into existence and am remembering this sentence, and am thinking of how to continue it, versus actually existing on the timescale of a human life.
There's another way to look at it: you can think of yourself as having just come into existence this instant, having inherited all memories and evidence of any previous existence.
Now you are blessed with the incredible gift of spontaneously coming into existence, plus a vast treasure trove of inherited experience (both good and bad) that you can explore and try to understand, and of inherited skills and knowledge that you can put to use however you choose.
I came to the same realization at about 12 years old. The mechanics were different than the Boltzmann brain. I saw it as that there are only two things that define the current universe: matter and energy. How could I know the universe wasn’t _just_ created? Every atom of my house, every atom of my body, created or placed in just the spot it is now, with just the energy it has now. My memories from five minutes ago are exactly what a brain that looks like mine would “remember” from five minutes ago.
Looking back, this realization became a feeling and this feeling became the backdrop for much of my growth as a person. Sometimes I was filled with fear. Sometimes I would shed all responsibility because it didn’t feel real.
At the best times, it fills me with a profound sense of agency, balanced with responsibility. It’s like teleporting into someone else’s body. Some things you choose to play along with, taking care of “his” mother. Other things you abandon, changing direction in your life as you need, despite what anyone might say.
