Z Wang

In the bone marrow, as in a barracks, there is a constant roll call. Rookie T cells wait for their first assignment, and for a while they all look alike. But with age, soldiers with a special mark on their uniform start to appear among them: the protein annexin A1. This mark is not a decoration but a brake shoe that keeps the cell from charging into battle.

In a hall of mirrors, a person walks up to a curved mirror and sees themselves now as a dwarf, now as a giant. Popular machine learning models behave in much the same way when asked to guess biological age from blood tests or brain scans: they stretch age at the extremes, underestimating it in older people and overestimating it in younger ones.