Adjectives in Neuter Singular
Also, I've come across прошло 5 лет; would it be incorrect to say прошли 5 лет?
Thank you very much for your help.
If the predicate is used in the Past tense with numbers, it is always going to be neuter.
В университет пришло три студента.
Было расследовано два уголовных дела.
However, there are some more complicated cases, even for native speakers. You can read about it in this article: