Research is pretty difficult because the original Greek text can be translated in many ways. Here are just a few examples I found on the 'Net:
"Reckon no man happy till ye witness the closing day."
"Count no man happy till he has passed his final limit free from pain."
"Call no man happy before death."
"Only when man's life comes to its end in prosperity can one call that man happy."
"Hold him alone truly fortunate who has ended his life in happy well-being."
Solon lived around 638 -559 B.C.; both years are somewhat questionable because such stuff wasn't recorded in Ancient Greece. Sophocles lived around 496 - 406 B.C., a contemporary of Herodotus. Again the years may be a bit fuzzy, but they are not wrong by 150 years, so it's perfectly possible that Sophocles quoted Solon from a pre-Herodotian source unknown to me.
An even earlier occurrence of the sentence can be found in Agamemnon by Aeschylus (525 - 456 B.C.). Of course that doesn't mean Agamemnon truly said such. He may have lived in the 12th century B.C., long before Homer wrote his Iliad, let alone Solon visited Croesus or Sophocles and Aeschylus wrote their plays. Anyway, it looks like Aeschylus is the older source than Herodotus and Sophocles.
And here we go again... One of the translations of Aeschylus' version I found reads, "When a man's life ends in great prosperity, only then can we declare that he's a happy man" (by Ian Johnston of Malaspina University-College, Nanaimo, BC, Canada). Other translations simply say, "Never call a man happy before death" or the like. Which again shows how difficult hunting up translated quotations can be...
Needless to say that Voltaire lived more than 2000 years later than those ole Greeks. He was an erudite guy, so most likely he just quoted one of the sources above.
As for the Chinese connection - of course it is possible that some folks in China came to the same conclusions as the Greeks. After all, there were wise people in both cultures. On the other hand, there were cultural connections between Ancient Greece and China - of course not very tight ones, but they did exist. WoZ' link is not very helpful (that's why he addedd "sic" to place himself on the safe side), so we cannot tell in which direction the saying could have migrated.
Signature: Hans Joerg Rothenberger