I know that in the Astral Plane children don't age (or more specifically no one ages), but is there anything regarding how children would age in the outer planes? I'm wondering specifically about a child native to the material plane and brought to the outer planes, not one born there, if that makes a difference.
This could change an important plot point in an adventure I'm preparing for publication, so "DM Fiat" won't quite solve this for me if I've missed something in one of the rulebooks, or an article or supplement. Basically the players are sent to one of the nastier outer planes to rescue a child from a terrible fate, and if they play their cards right, will learn that an equally terrible fate awaits her when she reaches a certain age back in the material plane. If the characters can just stay put in the outer planes once they rescue her, knowing she won't age there, it changes the ending of my adventure, and the motivations of the villains.
Although I'm writing for 5e, I'd be interested in anything from previous editions- if this wasn't addressed yet in 5e, I'll follow the last edition that has an answer. Also, I understand there may not be one answer to this question- different planes might have different rules: in that case, I could set my adventure in Carceri, The Nine Hells, or Gehenna without making drastic changes.
Best Answer
Unless otherwise specified, time passes normally on the outer planes (and any other plane)
As a general rule, you should assume that normal rules of reality apply except where it is clarified that they do not. The Dungeon Master's Guide describes the environment of various planes in the "Creating a Multiverse" section, and specifically of the Astral Plane on page 47:
This explicitly refers to creatures on the Astral Plane, and should not be extrapolated to other planes that are not the astral plane. If all of them had this kind of property, they would all state so in their descriptions, or it would have been clarified as a general property of the collection of planes in prior description. No such statement exists - so the conclusion is that time passes normally on other planes. Indeed, we're explicitly given that normally Astral-dwelling creatures will send children to live on other planes - often, but not exclusively, the material plane - so we must infer that time passes normally on at least some of the other planes of existence.
Referring to the 3e Manual of the Planes...
This is consistent with older material. In 3e, for instance, the Manual of the Planes handbook explores the matter of adventuring on the other planes of existence in some detail, and qualifies the different rules on different planes of existence by a system of "planar traits". The "timeless" planar trait, in this system, is a physical trait which is described so:
In this material the Astral Plane is timeless, but the inner and outer planes are explicitly described as generally having "Normal Time", meaning time passes as it does on the Material Plane, unless specified otherwise by the plane's individual description. None of the outer planes described by the manual actually do so, so the normal passage of time is the general rule. Even the chaotic realms of Limbo and the Abyss apparently follow normal time (though the Abyss gets a mention that there are rumours of a layer where time actually flows backwards). Besides the Astral plane, the Timeless trait usually only comes into play when dealing with unique/custom demiplanes.
There's no particular reason to believe that this particular rule of the planes should have changed significantly between editions. As written, they seem consistent with each other - the Astral Plane is explicitly timeless/ageless, but time passes normally in most other places.