[little jeremiah]

Historical fact. In the Bhagavad Gita, and with much evidence from the Puranas and the Mahabharat, caste or more accurately varna, is determined solely by quality of the individual. Someone who was born a brahman, for instance, but did not manifest the qualities of a brahman, was called “Brahma-bandhu”; a sort of snide term meaning “friend or relative of a brahman”. Same thing with kshatriyas or warriors. A warrior obviously cannot be born a warrior, he has to prove himself and develop the skills and mentality to be a warrior. And so on. I could quote verses if you want but not at the moment.

To the great shame of India, most people have forgotten this as it has not been taught. Fortunately there has been a renewed interest in the real teachings of Hinduism in the last few decades.

[SampleMan]

I'm curious, given how wealth and caste are interrelated, how an untouchable child became a warrior or a merchant, and how many low-life noble children actually found themselves in a low caste.

There is a difference between pointing out caste mobility between castes at comparable social levels and a truly open system.