I feel like Elmo isn't that quiet. He has appeared in at least two different segments teaching the concepts of loud and quiet.
Well, the big way Clash's Elmo voice has evolved is that the voice used to be much quieter in the beginning. Almost whispery, really. I actually miss that. It was a more low-key characterization (and a little easier on the ears, LOL).
It's hard to pinpoint when the voice evolved into its final form, because it happened slowly. From early '90s clips I have, by then it had lost the whisperiness, but it still had a softer quality than it has now. But by 1998 and Elmo's World, the change was complete.
Ernie may say he's six, but he seems older. Grover can't be younger than 16 in my mind with his employment history
Where did Ernie say he was six? (Apart from that "I'll be a doctor in 23 years" sketch where it's indirectly implied).
Personally I see Ernie and Bert as two guys in their early-to-mid 20s sharing a basement apartment, and Ernie just happens to be an overgrown kid (albeit a tongue-in-cheek one much of the time).
As for Grover, I see him, Herry and Cookie as being about 8, and his employment history can be chalked up to monsters being employable at much earlier ages than humans. Some monsters also get their secondary sex charactertistics at very early ages----the bearded and mustachioed Two-Headed Monster, f'rinstance. =)