Quoting Angel [Mariah's Mommy]:" It's actually under 3 months. General guidelines If your baby is under 3 months old and has a temperature ... [snip!] ... immediately. A baby this young needs to be checked for serious infection or disease. http://www.babycenter.com/0_fever_84.bc"
Until they're 6 weeks old, a fever can mean something scary like spinal meningitis. It normally requires a spinal tap unless there is a known virus going on.
Per my pedi (not some random baby site), I don't take them in unless they've had a fever for more than 3 days in a row or if it isn't responding to medication. A fever is part of the baby's immune system which is why it's ok for it to be that high. Giving them medication to lower it only suppresses their immune system.
I also have 3 kids, so you better believe that my kids have gotten sick well before the 3 month mark. It didn't require running off to the dr's every time they had a borderline fever.