Comic Strips

I can't come up with examples from modern popular culture, but I do remember the C.S. Lewis novels in the Narnia series often ending that way, as though the battles in Narnia were somehow less real. But those stories are a bit niche at this point.

Pretty clear that Digimon is one. Which links it to childhood and seems to have a sad/bittersweet ending multiple times. (though averted/retconned for 2 incarnations.)

Not entirely about lost powers but see the Growing Up Sucks trope (and the example subpages)

The concept of "childhood's end" will probably be clearly illustrated, with the now-grown-up character losing something that was fundamental to their happiness as a child.

The child may lose his guardians, Mons, or even his powers, if these all come with a time limit or are directly linked to his status as a child. For example, children are assumed to be wide-eyed, curious, innocent and trusting; adults are usually portrayed as pragmatic, cynical and set in their ways.

A recurring theme in Hayao Miyazaki's films

Sometimes it might be subtle, and sometimes the time limit is an open/common reminder such as Fairly Odd Parents (lose+forget fairies after turning 18, though it being a long-running show I don't know if they even actually ended on that note).

@elbarto777

Most magic stories about getting your powers before adulthood hits. Wednesday, Harry Potter, magic school, almost any magic series you search in both Netflix and prime, If you don’t start sensing it before you hit x age you’re not magical just ordinary. It really overplays on the crushing potential older people put on younger people too. This is pretty much all YA formula in books and what has been converted from YA books

Locke & Key has the forgetting magic when becoming an adult trope.

The Disney Channel series Wizards of Waverly Place has all the kids competing to keep their magic powers when they enter adulthood