How likely is it for a student to be expelled from university for online posts they wrote as a minor?
I'd say it's unlikely for a student to be expelled from university for on-line posts they made years prior. It may impact the student's reputation and may sway acceptance decisions in case of multiple potential candidates for a course or curriculum with limited capacity for students, but being expelled for such a thing would actually not reflect well on the university in question and may actually have repercussions for the institution.

Of course, it strongly depends on many factors; the nature of the posts, the association with certain groups or not, individual student acceptance rules and conduct rules of the university and what is considered "enough time passed" to reduce the weight of those posts.