Consequence is key to personhood. One can make the argument that, at least socially, you are 3 different people throughout your life, each with different levels of social responsibility. At first, you are a child, and social consequence is isolated to your core internal group or family unit, then you transition into adulthood, where social consequence places you subservient to your tribe, nation, or even planet, and eventually, you will once again become a 3rd individual in the dusk of your life, when social responsibility once again falls to the family unit, where you are subservient to your caretakers. The key is that the mistakes of an 18yo -can- affect a 50yo, through debt, prison, social shaming/othering, etc. One might argue that the 3rd person doesn't exist, and maybe even the 1st, too. But socially, we appear to distinguish a person from their responsibilities in these stages.

52 here. I'm a different person, year by year. Nothing radical, but I'm not the same man I was at 50 or 40 or 30 or 20 or 10.

Blindsight, by Peter Watts:

"I've been tweaked plenty. Change one more synapse and I might turn into someone else."

"That's ridiculous and you know it. Or every experience you had would turn you into a different person."

I thought about that. "Maybe it does."

Because I can trace a line of myself in time. I'm human, and I percieve time to be linear, so if I look back in time through my memories, I can see myself. Not only that, but there is hard physical evidence that I exist, and have existed since I was born in (what we label as) the year 1984

If, at any point, I was removed from the timeline I would cease to exist.

A sappling might not be a tree, but it's only that sappling that can grow into that, specific, tree. How it grows, and how it changes, is up to time. But even if it loses a branch, or gets scalded in a fire- it's still the same tree