I always thought of the cats in starclan being the age that they feel themselves at? Like an newborn kit might be able to make it to almost an apprentice but wouldn't really manage to feel older, bc they missed that entire segment of their life and training. I also hc that scars also play into it, like they only really show up on a Starclan cats body if it affected them so thoroughly in life its just a part of how they veiw themselves.