I am playing a Changeling Rogue in Eberron, currently working my way up the Stormreach underworld. Heavily focused on Charisma of course, mostly talking my way into / out of trouble. Like a supercharged Locke Lamora if you are familiar with the books.

I know a Changeling cannot assume a shape that is fundamentally different from his own (no Thri-Keen, Centaurs, Aaracokras), nor from a different size category (no halflings, gnomes). But could an adult Changeling impersonate a human child? Or would that count as "small" size category?

You can change the appearance of age but I would say any human child younger than about 7-10 would be considered Small (<4ft). For other races it might vary.

That is pretty much what I thought. Teenager should be fine, maybe a somewhat tallish, gangly 12 year old, but no toddler.

Which then opens up another question though. Could I change into a very young child of a creature of a larger size category? Like an ogre or half-ogre child?

It might take a bit of haggling with your DM on what age an Ogre child might be considered Medium and not Small or Large but I can't imagine why you wouldn't be able to do it.

The main limitation is that "You can’t duplicate the appearance of a creature you’ve never seen."  So if you've never seen a young ogre you probably can't become one.  Of course, since most other people have also never seen a young ogre, you could be an ugly human and say you were a young ogre without anyone likely to call you on it.

I have a variation on this question. I have a changeling that, in combat, was magically aged 30 years. He is currently presenting as a human male in his early 20s and, in game, has not revealed to his companions that he is a changeling. What is the effect on his form? Does he now appear as a 50-something human automatically due to the magic? Does he remain looking like a 20-something because that is how he set his appearance at some point, even though he is biologically a 50-something changeling?

Obviously, he can make himself look however he wants whenever he wants, but if he does not wish to reveal his shapeshifting abilities, what effect does the aging magic have on him, visually?

He's a changeling so it is his natural form that aged but he can still make himself look however he wants. That means the ageing effect won't change his masks unless he chooses for it to do so. If he is planning on hiding his shapeshifting abilities - and the others know he was aged - then he would want to age his human appearance to keep up the pretence.

But would it have automatically made his human face look older, or would it require a conscious effort for it to show a physical change?

I get that a changeling is control of their features, but a cut to the cheek makes a cut to the cheek whether or not the changeling wills it to be there. Such a weird situation.

Quote from Upchurch >>

But would it have automatically made his human face look older, or would it require a conscious effort for it to show a physical change?

I get that a changeling is control of their features, but a cut to the cheek makes a cut to the cheek whether or not the changeling wills it to be there. Such a weird situation.

 The age section says changelings can use their shape change to obscure their age, and the shape change ability says you keep your form until you end it as an action or become incapacitated, so my interpretation of the RAW is that their assumed form would not age until the changeling used their action to do so, but their real form would age immediately.  There’s some leeway always for a DM to rule differently but there’s no mechanic specific to magical aging that overrides the shape changer rules