Downloaded from here. There's an AF and TF version of the top and bottom and I have all four packages installed in total. The pants appear in the catalogue for both ages, but the top only shows up for adults.

I've had a glance at the CASP file in the package for the TF version of the top and I can't see anything which should cause it to not appear. I've also tried it in a freshly generated user folder but it still won't show up.

If anybody has any ideas (or has the time to test it for me in their game to see if it's not just a me problem) I'd be very grateful.

EDIT: Resolved! I tried one more time to get it show up, not sure what I did differently this time but now it's appearing in my game.

I checked both files. They both have the same CASP ID this is why you only see the AF one.
You will need to change it on the teen version.
If you have TSRW. Load it up change the description in TSRW this will force a new CASP ID. Export it and convert to a package.

This is the CASP ID they both have 0x3585B0F12D45C109. It will cause them to conflict.

Or you can try this.
Steps to Change CASP ID with S3PE Open S3PE and File: Run S3PE and click File > Open to load your clothing or accessory .package file.
Locate CASP: Find the tag named CASP in the list.
Use Grid Editor: Click on the CASP tag once, then click the Grid button at the bottom of the window.
Edit ID: Inside the Grid window, locate the Instance field. Change the hex code to a new, unique value to ensure it does not conflict with other items.
Commit and Save: Click Commit in the Grid window, then click File > Save in the main menu to save your changes.