<blockquote>
  <p>Ok to vote down newbies?</p>
</blockquote>

<p>Yes: when they don't show any research effort, are unclear, or not useful. Evidencing confusion about concepts<sup>&dagger;</sup> or making incorrect assertions shouldn't in themselves be considered reasons for down-voting a question.</p>

<p>It's wise to presume that new users are</p>

<ul>
<li><p>unfamilar with site norms,</p></li>
<li><p>inexperienced in asking statistical questions,</p></li>
<li><p>&amp; unaware that they can edit their question;</p></li>
</ul>

<p>&amp; therefore to explain in a comment how they can improve their question, if someone else hasn't already done so (that goes not only, of course, but especially, for new users). Those with the close-vote privilege will often prefer to vote to close a question when appropriate rather than down-voting it&mdash;an edit will automatically place a closed question in the queue for re-opening, whereas an edit to a down-voted question may well be overlooked (see <a href="https://stats.meta.stackexchange.com/a/4574/17230">https://stats.meta.stackexchange.com/a/4574/17230</a>).</p>

<blockquote>
  <p>Do you think this discourages new posters if they have their questions voted down? (Even if perhaps they received answers that are voted up?)</p>
</blockquote>

<p>I'd hope they're only discouraged from asking further poor questions. Constructive, polite, criticism should go a long way to mitigate any disappointment at a first attempt at a question's not being well received.</p>

<blockquote>
  <p>[...] is there some approximate metric, duration of account in days, or minimum rep before you believe a question should be voted down?</p>
</blockquote>

<p>No: most poor questions are asked by new users. Down-voting can flag the post for the attention of other users who may help (e.g. users with over 10k rep. can see a list of recent posts with a negative score at <a href="https://stats.stackexchange.com/tools">https://stats.stackexchange.com/tools</a>), &amp; I'd like to think it discourages premature attempts&mdash;perhaps also by new users&mdash;at answers. And it can make the difference between a question's being automatically deleted after thirty days, or hanging around for a year, or even indefinitely (see <a href="https://meta.stackexchange.com/q/5221/225179">How does deleting work? What can cause a post to be deleted, and what does that actually mean? What are the criteria for deletion?</a>).</p>

<p>&dagger; If this muddles the question to the point of making it unanswerable, it's sometimes good advice for the OP to take a step back &amp; explain the problem at hand without recourse to statistical concepts.</p>
