Disagree. Some of it IS well written. That stuff is almost never the 18+ stuff.
Anyways. On topic. The 13 is more cringy. Ignoring that a 31 year old has had time to, hopefully, practice their writing skill the fact is a 13 YO has a very... bad... idea of what constitutes 'good' writing. It's the kind of thing that results in edgelords who think darkness and evil are cool and that being 'misunderstood' is a rare thing that's never been done before and that their OC, who is pretty much just a petulant child lashing out in all directions and being evul because evuls is kewl but is really a good character at the end of the day and NOT something that's been seen six-billion times. The 31 YO stuff tends to just be some adult who wanted a cheesy schlock fest and, well, it might be cringy but not only is it usually at least slightly better written but it was never going to not be cringy. Looking at someone's personal pleasure piece and finding it to be little more than a personal pleasure piece is not shocking.
I WILL say, though, that there is a certain kind of 31 YO fan fic writer that's infinitely more cringy though and it's usually the one that basically turns whatever the source was into their own personal SJW fest and, if you dare to disagree, you're basically every thing bad under the sun and so-forth. However I feel that group is so exceedingly... distinct... that they should be mentioned separately from the normal writers in either age group.
A 13 year old doesn't know what they are doing. A 31 year old fanfiction writer clearly doesn't know what they are doing since they are writing fanfiction. ---