[Andreas]

This is really easy to achieve, if you don't rely on having an ACP option to do so:

In Template SHOWTHREAD replace

$quickreply

with

<if condition="$thread[lastpost] + X*86400 >= TIMENOW">
$quickreply
<else />
This thread is more then X days old, therefore QR is hidden.
</if>

Replace X with the number of days you like.