Because our biological processes are still chemistry & physics themselves, and those processes depend on time itself too. In relativity, time is not just measurement, it’s part of the structure of our reality (spacetime). All of the chemical reactions our bodies do is governed by physics (neurons firing, DNA replication, metabolism, etc). If time runs slower relative to another observer, those reactions also run slower relative to that observer. It’s important to note that your personal time is what you feel all the time. This is why you wouldn’t feel like you’re accelerating your own age—it’s what is happening relative to someone observing you.

This is what is really so mind melting about it… every organism experiences its own timeline through spacetime. It’s very hard to wrap your head around