If you are editing your profile, you will notice a pretty large button at the bottom of the page that says "Copy Profile to all Stack Exchange network accounts."
It looks like this:
If you click this button it will sync all your accounts with the one you are currently editing. So all the information you've entered in the profile you are editing will be copied to the rest of your SE profiles.
(I will say that I'm not sure how it works with empty fields. If a field is empty in the profile you're editing but full in another one and you merge, will the contents of the full one be emptied to exactly match the one you're editing? Suppose that's a question for later.)
As for your first question: unless you copy the data from one profile to another, then each profile will have its own data. As in, each stands for itself. If you have your age set on your StackOverflow profile but not set on your Meta SO profile, then that's how it stays until you copy the data from your SO profile to your Meta SO one.