He is supposedly as old as or older than Mundus, but that is no indication of his interest in it or its contents. It was not until his fall from the heavens, betrayed by Bocanadessia and The Seven that he was forced to interact with mortals on their level, and it was not until then that he patronized mortal men as a god, instead of as a passive observer.
It is clear that Genosus's returns to the Solar Temple atop Mt. Genosus coincide perfectly with the Eclipses. And on the rare occasions that he has successfully been beseeched not to take his holiday, he has been able to keep the sun in the sky at his own expense for that extra time, usually at the cost of a week or more of darkness.
What is unclear is whether or not Genosus IS the sun, or if the sun is some sort of vessel in which he resides when in the sky. The outline of the sun can still be seen during the eclipse, it's just black. Hence the "Order of the Black Sun."
Another important factoid is that Genosus is, even by his own word, under strict orders to protect this world in particular, and merely to observe all others. That is why The Seven chose Mundus as the world on which to spawn their accursed Dragons of Chaos, because that would be the most disruptive to the Celestial Twins' duties.
Genosus doesn't talk to people, but sometimes in the course of talking to one of the Pontifatrixes who come to commune with him, he mentions something of his nature that then ends up in the Solar Scriptures.
He has referred to "brothers" and "sisters" before, in the plural, meaning that there are more beings of his caliber than just Bocanadessia. However, there is nothing to suggest that he is in regular communication with them, or that they even exist in this world. They might not even still be alive.