1what is the structure
enters and leaves
that regulates what
the cell ?

If I remember correctly, its "the plasma membrane".

Explanation:

The plasma membrane is the outer part of the cell and it can control what ever leaves the cell.

age range would be the answer

answer : haversian

[explanation: -(compact bone is organized as parallel columns, known as haversian systems, which run lengthwise down the axis of long bones. these columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone)]