The body is like an engine that gets a limited supply of fuel vouchers. Every heartbeat is one voucher spent. A mouse burns through its vouchers quickly, an elephant slowly, but by the end of life both spend roughly the same amount. That's why aging is better measured not in years, but in the number of vouchers spent.

The internal clock doesn't tick in seconds, it ticks in heartbeats, but each beat has its own price. If the heart beats faster, or if each beat costs more, the clock runs faster. This explains why people of the same age age differently: for some, the clock ticks faster, for others, slower.

When the body repairs damage, the repairs may not keep up. If the repair rate drops below a critical level, the healthy state disappears, and aging accelerates sharply, like an avalanche. This threshold can be calculated, and it explains why decline speeds up at the end of life. Testing against data showed that this internal clock predicts aging more accurately than age alone or heart rate.