The sentence in question (on the test I composed) is:
Somehow, it seems to worry you when your parents _______ (grow) old.
I expected my students to use the present simple because the present simple is used in temporal clauses. In this instance, "when" indicates a period of time. This is standard usage.
Yet, most of students answered with the present continuous. Upon more careful consideration, it does seem plausible with the present continuous if we apply the rule that says "when we talk about changes happening around now, especially with verbs like rise, fall, grow, etc. (Reference: English Grammar in Use by Raymond Murphy, Fourth Edition, Cambridge University Press 2012)
Somehow, it seems to worry you when your parents grow old.
Somehow, it seems to worry you when your parents are growing old.
The more I read the sentence with "are growing", the more it grows on me (pun intended).
I seem to have lost my native-speaker feeling after reading more "are growing" answers than my preferred "grow" answer. In addition, I know that the present continuous aspect has crept into the language in the last decades, and I'm just hating it (My attempt at wit.).
I always want to be fair to my students and, therefore, am asking for help from native-speakers. Consider that my students are training to be translators, so accuracy is essential.