Think at oldsters: what will they feel with a flashing Red in front of their eyes ?
Julian answer have my preference (instead of mine who was an Error Reporting Label at the window bottom and a beep, eventually).
Entering dates:
I do not liked that too much, but in a project where the user enter birth (death and other dates), I use three PopupMenus:
a. Day # (1 thru 31 max),
b. Month #/Name (I forgot, 12 entries),
c. Year (here lies the implementation limit (100 entries: a centurystarting the current year [2019] and ending 100 years ago [1918]).
I used a Date Picker Control for an Agenda date selector, but I have no feedback from the oldsters what they prefer.
We “oldsters” prefer what anybody prefers. We want to know where and what we did wrong and how we can do better.
How i make my users aware of an issue depends on the kind of App and Context, not how “old” they may be.