[kh99]

You could add it to the join date, like the age is done with the birthday. Create a plugin using hook userprofile_prepare and code like this:

if ($field == 'joindate')
{
    $this->prepare_joindate();
    $jd = vbdate('Y', $this->userinfo['joindate']);
    $td = vbdate('Y', TIMENOW);
    $years = $jd - $td;
    $this->prepared['joindate'] .= " (Member for $years years)";
    $handled = true;
}

Obvious you could expand that to go to months if they joined less than a year ago, etc. (BTW, look in includes/class_userprofile.php to see what's going on).