I tried using youth:ancestor-or-self::section[last()]/folder, but that gives me a validation error.

its the element name in the namepsace,not the axis

ancestor-or-self::youth:section[last()]/youth:folder

(guessing, as you didn't show the input)

on the otehr hand rather than going all the way up each time, you could
just start from the other end

/a/b/youth:section/youth:folder

if th etop level section is at some known position relative to th eroot
of the document.

As its a constant you can make it a global variable

<xsl:variable name="path" select="/a/b/youth:section/youth:folder"/>

then you can just use $path

ie replace by

<a href="{$path}/{youth:url}">

David