In my experience (having taught a fairly even balance of adults and children), with all things equal (such as enthusiasm, commitment to practice etc) adults tend to progress at least twice as fast as children.
Or perhaps I've just had a rather gifted bunch of adults! Who knows
From my observations I would say the odds are stacked in favour of the adult. Better problem solving skills, better math skills, better reading skills...they all help adults pick things up a little bit quicker. I've never had any difficulty with particularly unco-ordinated adults (no more than most children starting), so I wouldn't say that a lifetime of not
using your fingers in a certain way makes you any worse off than a developing child starting to use their fingers that way.
The only disadvantage I have seen on numerous occasions is PATIENCE. Adults tend to have had much more exposure to music and music performances. They have a much more concrete idea of what is good
, and they tend to want to be good NOW. I have had quite a few adult students who have been progressing incredibly well, far beyond my expectations, who then come to me one day all disheartened because they feel like they're progressing "so slowly" and asking if they're "a lost cause!" I ALWAYS encourage my students and tell them how well they're progressing, so I don't think it's any failure to communicate this on my part. I think adults tend to expect too much from themselves in too short a time. If this can be overcome, adults usually do incredibly well.
As for how long to get to grade 6? Well...it is like a piece of string! But, in my experience, 3 to 4 years tends to be the average for adult learners if they are consistent and thorough in their practice.