The progress of a human being through life might be thought of as a mostly gradual succession of changes from the ovum to the grave.

But if you wake up one morning, look in the mirror, and wonder when you suddenly grew so much older, you may not be imagining things.

According to recent research into the molecular changes associated with aging, humans experience two drastic lurches forward, one at the average age of 44 and the other at the average age of 60.