One of my production reds (Similiar to golden comets/sex-links) just practiced her egg song this morning, and she's only 16 weeks old! I'm expecting her to lay in the next few days. So your comets can lay pretty early, or very late depending on the chicken. I've never had a plymouth rock, so I can't answer that, sorry!
Check out the topic I've linked below, called "How old was your chicken when she laid her 1st egg?" The very first post has a link to a spreadsheet where BYC people have posted the earliest & latest their hens have laid, by breeds. You might get a better picture of when yours will lay.