Why is adult drug dosaging different from that of a child or baby?

Children's body system is not fully developed like the liver's enzymes that metabolize most of the drugs and kidney as well and other metabolism... pathways of drugs in the body. But for adults they are developed well and system can handle the drugs even according to their increased weights.
For the elder ones dosage is reduced as their system are exhausted and not working so well.
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Which do you prefer? Do you prefer to go on leave or you would prefer to work and be paid instead of embarking on leave?

Embark on leave since our bodies need rest so as to work efficiently or else the work we will be doing will be burdensome and we would be unproductive

Why is Tuberculosis treated for such a long time (6months-2 years)?

To ensure the latent form of the mycobacterium is eradicated fully to prevent relapse.