What is the punishment for plagiarism as an adult?

It is copyright violation (not plagiarism as such) that is punishable by law. In practice, it is usually treated as a civil matter. It can, however, be treated as a criminal matter, especially if you make money from the copyright violation or do so in order to avoid buying the book(s), music, etc. For example, there have been cases of schools that have been fined for photocopying sheet music without permission. (Sheet music tends to be quite costly: hence the temptation).

Plagiarism - that is pretending that someone else's Words, photos, diagrams, etc. are your own, for example by failing to give the source - is highly unethical and can damage your reputation even if no copyright violation is involved. No college wants a lecturer who is dishonest ...