As is often the case, things get murkier when you dive into the comic books. While Thor and Thanos are still usually presented as ancient beings, both characters’ ages fluctuate, depending on the writer and whatever the story dictates.
A depiction of Thor’s birth takes place in “Avengers: 1,000,000 BC” #1, but even with a date in the title, it’s not a ton of help. As the name suggests, the book follows the earliest iteration of the Avengers (which Odin is a member of) a million years ago. However, while we have a start date for the comic, Thor’s birthing scene is said to take place “many years later,” and that’s all we get. Thor has graced comic book pages for over half a century, and the best approximation of his age would be “many years” less than 1,000,000.
Unlike his MCU counterpart, comic book Thanos is oftentimes portrayed as older than the God of Thunder. In 2020, we see in the pages of “Avengers” #38 that Thanos is at least 1,000,000 years old. The book features a child version of the Mad Titan interacting with Odin’s ancient Avengers a million years ago. No age is given, but he looks to be in whatever the Titan equivalent of his teens is.
So, while it’s harder to pinpoint a specific age for the comic book versions of Thor and Thanos, and their lifespans fluctuate wildly, the print versions are decidedly older than what we see onscreen.