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