This thread gives the definitions of each of the Usergroups. One thing you may not have noticed is that not only does your post count figure in it, but also the length of time you've been a member. So you won't have the full privileges of a Young Cat until you have been 30 days in membership, even though you do have the required posts. Looks like you'll have your 100 posts for Adult Cat well before 30 days, too, but those privileges will likewise not come into effect until then. And so on...
However, none of that will have any effect on your access to the PM system. For the safety of minors, private means of contact are not available to members under the age of 18. If you have questions about how the site operates, you might want to post in the "Forums How-To and FAQs" Forum, and I'll move this thread there as well.