  We report the X-ray detection of the brown dwarf (BD) companion TWA 5B in a
$\simeq 12$ Myr old pre-main sequence binary system. We clearly resolve the
faint companion (35 photons) separated from the X-ray luminous primary by 2
arcsec in a {\it Chandra} ACIS image. TWA 5B shows a soft X-ray spectrum with a
low plasma temperature of only 0.3 keV and a constant flux during the 3 hour
observation, of which the characteristics are commonly seen in the solar
corona. The X-ray luminosity is 4$\times10^{27}$ erg s$^{-1}$ (0.1--10 keV
band) or $\log L_X/L_{bol} = -3.4$.
  Comparing these properties to both younger and older BDs, we discuss the
evolution of the X-ray emission in BDs. During their first few Myr, they
exhibit high levels of X-ray activity as seen in higher mass pre-main sequence
stars. The level in TWA 5B is still high at $t \simeq 12$ Myr in $\log
L_X/L_{bol}$ while $kT$ has already substantially cooled.
