To say ‘I am fifteen thousand days old’ seems like a strange affectation—as if I am trying, Methuselah-like, to exaggerate my age. But it's nothing but the truth. (Indeed the truth is that I have lived for more than fifteen thousand days…)
Why should it seem such an enormous age? I suppose it touches on the cliché of parenthood, that the days are long but the years short: forty years is a heartbeat (in the larger scheme of things) but fifteen thousand days draws your imagination into the sticky specifics of so many individual slots of times: getting yourself out of bed fifteen thousand times; fifteen thousand afternoons as the sun through the window pushes a parellelogram of light slowly across the carpet; fifteen thousands sinking, blushing suns.
I might hope to live thirty thousand days, or even thirty-five, conceivably—just—as high as thirty-eight or nine. Thinking about it like that puts it in the realm of the stone age—many people die in their teens (of thousands of days); some lucky few make it into their late twenties or thirties. Nobody makes forty. Life fleets.