People are stuck in the role they were born to, but it seems like there are no artificial barriers to advancement

In the lower three classes, people are cloned in order to produce up to 96 identical "twins." Identity is also achieved by teaching everyone to conform, so that someone who has or feels more than a minimum of individuality is made to feel different, odd, almost an outcast.

https://www.huxley.net/studyaid/bnwbarron.html

I'd argue that intentionally cloning people with limited abilities and then raising them to stay in thier lane is an artificial barrier...