[The hot start model suggests Enceladus began as a mixed-up ball of ice and rock that contained rapidly decaying radioactive isotopes of aluminum and iron. The decomposition of those isotopes -- over a period of about 7 million years -- would produce enormous amounts of heat. This would result in the consolidation of rocky material at the core surrounded by a shell of ice. According to the theory, the remaining, more slowly decaying radioactivity in the core could continue to warm and melt the moon's interior for billions of years, along with tidal forces from Saturn's gravitational tug.]

From: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=729

We have comets today that were born (and/or achieved active interiors) when our solar system formed. Enceladus may not be so different (an active KBO or even an old Oort Cloud member).