Growing Up Sucks/Analysis
In more fantastic shows, the metaphor may be even more blatant. The supposedly inevitable loss of their adaptability, or belief in magic, can mean even the most bright-eyed Kid Hero can become just like all the other generic adults just by reaching a particular birthday. Most of these former protagonists will end up just like the parents they were so determined not to grow into. Truth in Television, perhaps, but a bit heavy for a cartoon.
Attitudes towards this vary. Some see it as a rich source of Character Development, and allowing more scope for Story Arcs as opposed to the formulaic shows of the "immortal" characters. Others see it as pessimistic and rather sad—it can be argued that cartoons are meant to be an escape from reality, and bringing up the nature of time is enough to depress most adults, never mind children. Even more, becoming an adult is supposed to be a good thing, making you a Real Person who can Do Stuff; why are we telling kids to avoid it at all costs? It's also breaking to the story; by implying the characters were not real, but not wanting to flatout admit it because it was All Just a Dream tends to irritate people. So they were...sorta real; but not anymore, and abandoning them now is growing up, really.
It might be said that actually this Trope is not directed at kids at all, but instead to and from adults, a wistful longing for the ideal of childhood as seen most clearly in movies like Big and Freaky Friday.
This also includes adults as well, who are made to give up magical things in order to live in the "real world".