It's a complex process. The group follows various leads, explores different areas, and eventually stumbles upon Sophia after much hardship and determination.
Carl and Sophia are indeed deceased in The Walking Dead comics. Their fates were determined by the story's progression and the challenges the characters faced in that post-apocalyptic world.
Sorry to say, Sophia isn't alive. The plot and circumstances in the comic didn't allow for her to continue. It's a harsh but integral part of the comic's narrative.
Sophia's age in the comics isn't explicitly stated. But based on the story progression and her appearance, she seemed to be in her early teens, maybe around 11 or 12.