I don't think his death age signified anything in particular but the absence of a biography of Jesus between the age of twelve to thirty, if memory serves right was a direct parallel with Horus myth.
When looking at the mythos behind a story it is probably best to know what the key signifiers were in the first place. To fulfill solar myth the hero's death had to be at the time of "the cross" i.e. when the Sun's path (the ecliptic) crosses the equator at the spring equinox in the northern hemisphere.
In the matter of Sun gods and their offspring the moon does not usually figure and neither does thirty three years. The moon gods were usually feminine-- although in the Levant there were many exceptions including Yahweh who was a Canaanite male moon god, with his regular curved horns relating to the crescent Moon. The other element for the Moon would be its shorter periodicity not relating to the whole astronomical cycle of the solar year which would be the Sun god's rightful period.
Another reason to exclude the Moon as a type for JC was that the son of the Sun god was "the exact representation of his very being". How perfectly true this was, as each morning it was witnessed by the ancients who saw in a glorious winter sunrise over the Nile, the rising Sun god and his perfect reflection in the water below.
The fact that the Sun 'dies' for three and a half days I suggest is applied as a literary device to simply signify that the hero character who goes through this process (Odysseus and JC etc) was a mighty god like person. He is not required to do it as the same time as the actual Sun does.