Adding a child element in XSLT

I recently had to edit a big XML file, and add a child elemnt to every element within. To simplify matters, say I had something like this:

<?xml version="1.0" encoding="UTF-8"?>
<rooty xmlns:ppl="some_identifier">
  <ppl:person>
    <age>42</age>
    <name>bob</name>
  </ppl:person>
  <ppl:person>
    <age …
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