There's never any indication that humanoid Cylons don't age. They are susceptible to disease and radiation, just not as susceptible as humans are. In the Battlestar wik, in the article about humanoid Cylons, it says:
The bodies of humanoid Cylons have stamina and strength above human average, and are designed to destroy or resist commonly dangerous human diseases, although they are not immune from all contagions. In keeping with their desire to be a better human, however, the Cylons did not or could not further improve on other characteristic design flaws of the human body.
So not everything was improved upon. The Cylons could live forever because they would download their personality and memories from one body to a resurrection ship on death, only to have those memories and that personality transferred into a new body. If there was no resurrection ship available, or they were unable to download for some reason, then that individual would die. And there is never any indication given in the series that humanoid Cylons don't age. And, considering that Adama and Tigh knew each other for decades, and Adama found it hard to be believe Tigh was a Cylon, that indicates there were no clues for Adama to put together, even after knowing, that would support that - so Tigh must have aged like a human.
Considering just the odds, even if Cylons didn't age, there's wear and tear, injuries, and so on. If they were ageless, at some point disease would be an issue or starvation, or any one of many other things over the next hundred and fifty thousand years.
But also, with humans that were barely creating their own civilization at the time, if there were humans that lived a long time, that would lead to stories and myths about immortals who have great powers and can't die....
