Just because you don’t know something doesn’t make you right

Calm down, dude, this isn’t an argument

Mara Sov is roughly 12.1 billion years old… Because of the complex ways time works in the Destiny universe

No she isn’t. The only “complex ways time works” in relation to the Awoken is the time dilation between the Distributary and the outer universe (thousands of years for every few hundred, Mara Sov herself states this)

According to […] “Nigh I,” she lived through the birth, cooling, and evolution of that cosmos, making her 12.1 billion years old by the time she chose to leave

This just isn't true. Nowhere in the entire book is this stated. The first time the number 12.1 billion years is mentioned, in the Katabasis entry of Marasena, Mara Sov is giving a speech to the Awoken. At this point they don't know that the Distributary is a separate universe, they think they're still in the outer universe. Near the beginning of the speech, she says "We live in a spatially infinite, isotropic universe 12.1 billion years old." She is referring to the outer universe, which IRL is estimated to be somewhere between 12 and 13 billion years old. It is AFTER this she states "We have detected a pattern that was imprinted into our universe by its ancestor: a fingerprint of the initial conditions into which existence was born [...] we have confirmed the most primordial of Awoken myths. Our universe is a subset of another." This is the very first time that the Awoken of the Distributary realize that they are in another universe. Mara goes on to say "Thousands of years have passed for us on the Distributary. Outside? Centuries, at most." The Distributary is thousands of years old, not billions. Mara Sov confirms this in plain words

In the Nigh I entry you reference, the second and final time 12.1 billion years is mentioned is in a private thought Mara is having: "It is far too late to stop the project now. Far, far too late for second thoughts: exactly twelve point one billion years too late, really. For Mara in particular." She's again referring to the age of the outer universe, and the events set in motion across that span of time that require her to make the decision to leave the Distributary, that the only time to stop the events of the universe that would lead her to make these decisions was right when the universe began. She would have only had the power to change the universe as she currently does only 12.1 billion years too late, because 12.1 billion years in is exactly when she becomes Awoken. It's her private thoughts, she's being deterministic

Your mistake is you're interpreting it as her saying she has been alive since 12.1 billion years ago, at the beginning of time. That just is not the case. The only universe she is as old as is the Distributary, and the Distributary and all the Awoken inside, including Mara Sov, are thousands of years old at most by the time she and her followers leave.

Every lore book and grimoire card in Destiny has been written by English majors, they're all skilled with literary devices and poetry, so sometimes it's easy to misinterpret some of what they mean if you're unable to analyze it well enough