I think you will find that many who teach full-time, multiple instruments in particular, will start students from age four as a matter of routine, with great success. I have had students of this age who can use normal guitar chord shapes ( on a half-size guitar) with no problem, and in some cases play keyboard as well. Note reading is not a problem, either, in children of this age group, because they only have eight note names plus the #'s and b's to handle, and most kids in my experience can read words to some extent at this stage thanks to their parents.
Most of the greatest instrumentalists in recent history started learning around this age- very few, as suggested above, started at 11.