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It is not so much that they "outgrow" it but some can do what is called SEROCONVERT which means their positive test reverts to being negative as they age and handle their own antibodies in the body.
Some do, some don't.
Your adult cats should have two boosters separated by 3 weeks. I would wait for about 6 weeks before introducing the kitten to the cats.
If your kitten does not convert...the kitten will need to stay inside and many times they can live several years before the virus wins.
Let me know if you have further questions.