If, at some frame, you’ve saved a particle shape’s attributes for its initial state, rewinding an animation does not return the age of the particles to 0.

Suppose you’ve created a particle grid having an opacity attribute that fades gradually as the animation plays. You stop the animation at some frame where you decide the grid’s opacity looks good as a starting point for the animation. You then select the grid. From the Dynamics menu set, select Solvers > Initial State > Set For Selected to cause the current value of the object’s attributes—including age—to become the initial state values.

If you rewind the animation, the age of the particles in the grid is equal to age at the time you chose Set For Selected. The age of the particles therefore is not equal to 0 when you rewind the scene.

See Understand initial state attributes for more details on initial state attributes.