Well since my last answer wasn't understood it seems I'll have to spell it out. Jesus died at 30 Years because there exists prophecy in Scripture that Jesus would die after 5,500 Years from Adam.
This timeline is embedded within the creation week of Genesis 1:1-2:3 which represents an historical timeline of 7,000 Years. This is because God told Adam:
"but from the Tree of the Knowledge of Good and Evil you shall not eat, for in the day you eat if it you will surely die."
Genesis 2:17 (emphasis added mine)
And when did Adam die?
"So all the days that Adam lived were 930 Years, and he died."
Seeing that Adam died in the day he ate of the forbidden fruit: each day of the week represents a period of 1,000 Years. So then God from the very beginning wrote that there would only be 7,000 Years of recorded history.
And we see that Adam was created in the middle of the 6th Day while the animals of the earth were created in the beginning of the same day (Genesis 1:26-28); and, while not explicitly stated we can draw this conclusion from the fact that instead of God creating everything instantly He spread out everything over a 6 Day period, and since this is so we may also safely say that it took God some time to form all of the animals; and, and since this is so it is not hard to believe that God gave Adam dominion over all the animals and the earth at the end of the 6th Day.
All this means that after 5,500 Years Christ would die and rise from the dead: this we see in the creation week for exactly 3 Days and 3 Nights prior man did not exist and God was creating the earth and the plant life from the ground (Genesis 1:9-13). And when Adam was created from the dust of the ground: this is a shadow of Jesus Christ's resurrection from the dead. Adam is a type of Christ for he and Eve are the only people to not have earthly parents give birth to them, therefore this makes Adam the Son of God and the first type of Christ who is the new Adam. And after 6,000 Years Christ would return to rule the earth for 1,000 Years.
We also see proof of this in the dimensions of the Ark of the Covenant, in cubits, which add up to 5.5 Cubits (Exodus 37:1) which are 5,500 Years before Christ would come in the flesh and die for our sins. The final proof of Christ dying 5500 AM is in Daniel's 70 Week prophecy where he gives us the exact year Christ would die:
"So you are to know and discern that from the issuing of a decree to restore and rebuild Jerusalem until Messiah the Prince there will be seven weeks and sixty-two weeks; it will be built again, with plaza and moat, even in times of distress. Then after the sixty-two weeks the Messiah will be cut off and have nothing, and the people of the prince who is to come will destroy the city and the sanctuary. And its end will come with a flood; even to the end there will be war; desolations are determined."
So this means that Daniel is beginning his count from the time of Cyrus the great as he foresees the delays in temple construction, being done in times of distress, and therefore says this will happen in a 7 Week/49 Year period; that is why he says after 62 Weeks/434 Years the Messiah will die (see interlinear Hebrew to confirm that Messiah, and not Prince, is used).
Now if we count all of the years from Adam to the death of Christ according to the longer chronology in the Septuagint/LXX then we get 5,500 Years; but according to the shorter chronology found in the Masoretic Text there are 4,034 Years for the same period. Here is a breakdown of the LXX reckoning:
- 2,242 Years (Adam to Flood)
- 1,247 Years (Flood to Abraham)
- 430 Years (Abraham to Moses)
- 511 Years (Moses to Saul)
- 517 Years (Saul to Babylon)
- 70 Years (Babylon to Cyrus)
- 483 Years (Cyrus to Christ)
Cyrus began his reign in the year 5017 AM and 483 Years later Christ would die 5500 AM. We may verify this to be true when we look at the reigns of Augustus and Tiberius. Augustus came into power in the 184th Olympiad while Christ died in the 202nd Olympiad, which is also the 15th Year of Tiberius. Therefore this is a span of 72 Years which means Jesus was 15 Years old at the death of Augustus and the 1st Year of Tiberius; and died at 30 Years old in the 15th Year of Tiberius. This then would mean that the Ptolemaic dynasty lasted for only 280 Years; Alexander the Great ruled for 12 Years from the 111th Olympiad to the 114th Olympiad; and from Cyrus to the end of the Persian kingdom is 119 Years. And 1 Olympiad = 4 Years.
So it is clear that God planned for Jesus to die at 30 Years old 5,500 Years from Adam. And when you check the time of His death against history it all adds up. And finally this prophecy was given cryptically to Adam and Eve when they were in the garden of Eden:
"And I will put enmity between you and the woman, and between your seed and her seed; He shall bruise you on the head, and you shall bruise Him on the heel.”