Aged with one syllable seems to be limited to phrases with a number of years
- A man aged 46 was arrested yesterday.
and when referring to non-human things (aged cheese, well-aged beef, unaged wine).
Generally speaking, adjectives formed from regular participles, like wanted, believed, added, whispered, etc, follow the pronunciation rules for the {-ED} past tense morpheme.
{-ED} is pronounced
- /-əd/ after dental stops /t/ and /d/ (because it's impossible to say final /-td/ or /-dd/)
- /-t/ after other voiceless sounds /p k f θ s ʃ/
- /-d/ elsewhere, including after vowels
In the case of aged, the final sound is voiced, and therefore /-d/ is possible, but it's an affricate /dʒ/ formed from a dental stop, so the epenthetic shwa is possible, and occurs in some circumstances, as FF notes in his answer.
In the case of clothed (clothed all in white), either /-d/ or /-əd/ is possible because /ð/ is voiced but also dental (final /-ðd/ is hard to pronounce), though the two syllable version is usually marked with an accent to distinguish it (clothéd all in white). See Pratchett & Gaiman's Good Omens for some examples.
In the case of marked, I think Jonathan has it right -- markedly gets the epenthetic shwa to separate the /-ktl-/ cluster, with the /t/ voiced to /d/ by the preceding vowel.