Researchers with Stanford University have published a study revealing that physical aging is not a smooth process, but rather something that happens in what they describe as a ‘herky-jerky trajectory.’ Using blood tests to look at specific proteins, the researchers found that human aging involves three distinct turning points, the first starting in one’s mid-thirties.

Proteins in the blood can provide clues about a person’s health and, according to the new study, show changes that allow researchers to estimate a person’s age. The study found that the levels of many proteins in the blood change with age — something that may cause, rather than just reference, the process of physically aging.

Read more at Slash Gear.