If he is dead (let's say that he died, and Frank killed him too), but how come he grew from 10 year old in a hanging scene to his present age?
Harmonica is dead.
While it is fun to speculate on this kind of thing, I don't buy this idea for a minute. Some slight evidence may be advanced to support it, but not enough that you can duck being accused of reading too much into too few details.
Because it has something to do with Death...