And it talked about arranged marriages and houses, and I was thinking, really, this is rather feeble for a caste system. It's not as if people could duplicate the effects of the magic, especially the powerful ones.
In real life, aristocracy were more flexible. In every aristocratic era, there have been complaints about social climbers and johnny-come-latelies and all that. Which shows that there were people rising into the aristocracy in every era.
When you can indeed tell the difference between them, there is going to be a caste system of unprecedented rigidity. (And one's little accidents on the side are going to need rigourous looking after.)