When it comes to my characters, since most don't know the other, I tend to make their ages randomly without any set time. Except for a certain few that are set in certain times such as the WWII era (my character,
Baliff), and Balto's time, etc. As for any pups or whatever, no, I don't use timelines, really.
When it comes to the ages of the characters, I think Little Creek is somewhere from 19-22. After all, Spirit did call him a boy, so I think he's a fairly young adult. The colonel, judging from the slight wrinkles in his face, is probably somewhere from 32-38. Spirit, I believe is also a quite young adult. I think Spirit would be about 4-5 which would be somewhere from 20-24 in human years. Rain would probably be either 3-4 which is somewhere from 18-20 in human years. I think you're right in that respect (she's probably younger than Spirit, though not that young). And finally, Esperanza I would probably put somewhere from 13-17 which would probably be around 43-53 human years.
Interesting questions!