Why do older people use the word “dinner” for “lunch” while younger people use “dinner” for “supper”? When did this change, or is this a regional phenomenon?

I keep having to explain this. It must no longer be taught.

OK

Breakfast is ‘break the fast’ and a fast is a time of not-eating. You don’t eat while you are asleep, so ‘breakfast’ the meal is always the first meal of the day.

Lunch is at midday.

Supper is the LAST meal of the day. There used to be a time when people ate four times a day — and supper is always the last and latest meal.

DINNER however, is not tied to a time of day.

DINNER is the LARGEST meal you eat during the day.

So sometimes ‘dinner’ is at lunchtime, and sometimes ‘dinner’ is at suppertime. Because breakfast time and lunchtime and suppertime are TIMES, but ‘dinner’ is a size of meal.

That said — in CURRENT American culture, the largest meal of the day is often at suppertime, so supper = dinner to a LOT of people, even though that is not exactly true. So starting — probably — around 40 or 50 years ago, people started saying ‘dinnertime’ and they meant ‘in the evening’.

I heard that sometimes when I was young (I am old enough to remember this) but it was still pretty clear that DINNER was simply AT suppertime, not that supper was invariably dinner.

In fact, on Sunday, DINNER was specifically at LUNCH time, and ‘Sunday dinner’ was a real event, which took place sometime between noon and one o’clock.

Thanksgiving ‘dinner’ is also typically in early afternoon (‘lunchtime’) as is ‘Christmas dinner’.

DINNER is your largest meal. Supper is in the evening.

If your largest meal of the day is always at the last meal of the day, then your supper is your dinner.

Edit to add: apparently some people like a reference ALSO, rather than trust that someone who answers knows whereof they speak. So here:
https://www.merriam-webster.com/dictionary/dinner