Why do girls start vaginal discharge at puberty?

Vaginal cleaning is just a 'nice' way of putting what is occurring. It is changes of the mucous (similar to the stuff in your nose) from your uterus.

At any rate, you don't have it when you are young because it is a process of becoming reproductively viable (able to conceive). Further, this happens at 'puberty' because of hormonal changes, and surges of primarily estrogen and progesterone which cause the development of mature reproductive organs. Which for women, involves the shedding of the uterine lining, AND the development of mucous and excretion of this mucous from the uterine lining. It isn't 'cleaning' like you clean anything else.. it is a cycle of ridding itself of "Older" cellular matrix with new. Which all correlates with reproductive viability and womanhood, etc.

The vagina as with other parts of the body do not inherently need to be cleaned. Cleaning, and hygiene is something WE have invented. The body is designed to work just fine without all of our hygienic practices.

Hope that helps,

You start it at puberty because you are ready to have period blood. Having the blood means you vagina needs to keep flushing stuff out. The blood doesn't exactly make it clean in there.