How to return values from sub-template?

Sorry for this newbie question but I didn't found a hint in tutorials on how
to return
back parameter values from sub-templates. Assume the following situation.
I want to pass back the boolean (or other values in other scenarios) value if
the attribute "age"
exists in a node "myelement" with the attribute name="karl".

Yes, I know, THIS sample could be coded easier within
one template but my sample is only a simplified version for a complex
scenario
where two templates are required.

Again, how can I access the value of the variable myval from the calling
template?

Thank you
Matt

<xsl:template match="/">
   <xsl:call-template name="subtemp">
     <xsl:with-param name="parm" select="//myelement[@name = 'karl']" />
   </xsl:call-template>
   Age exists=<xsl:value-of select="$myval" />  <!-- does not work -->
</xsl:template>

<xsl:template name="subtemp">
  <xsl:param name="parm" />
    Hello
  <xsl:variable name="myval" select="exist(@age)" />
  return $myval
</xsl:template>

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