What is the earliest example of proto-writing that dates to the Bronze Age?
Proto-writing
cuneiform script
The transition from proto-writing to the earliest fully developed writing systems took place in the late 4th to early 3rd millennia BCE in the Fertile Crescent.
The Kish tablet, dated to 3500 BCE, reflects the stage of "proto-cuneiform", when what would become the cuneiform script of Sumer was still in the proto-writing stage. 
By the end of the 4th millennium BCE, this symbol system had evolved into a method of keeping accounts, using a round-shaped stylus impressed into soft clay at different angles for recording numbers.  This was gradually augmented with pictographic writing using a sharp stylus to indicate what was being counted.  
The transitional stage to a writing system proper takes place in the Jemdet Nasr period (31st to 30th centuries BCE).
A similar development took place in the genesis of the Egyptian hieroglyphs.  Various scholars believe that Egyptian hieroglyphs "came into existence a little after Sumerian script, and... probably [were]... invented under the influence of the latter...",[7] although it is pointed out and held that "the evidence for such direct influence remains flimsy" and that "a very credible argument can also be made for the independent development of writing in Egypt..."[8] (See further Egyptian hieroglyphs).