Adults aged 18-21 were given the right to vote in 1971
Why by Mimiwhatsup: People held enslaved people as workers and relied on them to operate. Especially for picking cotton they even bred slaves and sold them as high prices.
It is now believed that the largest contributing factor to the decline of the Indus Valley civilization was an outbreak of disease--since there were no treatments and the density of the civilization led the disease to spread very quickly.