Hi @Jayden1,
Welcome to the Bunnings Workshop community, it is fantastic to have you with us.
If the decking boards are only 8-12 weeks old, then there shouldn't be any need for you to sand them prior to application of a stain or oil. The 8-12 weeks weathering process is so that the natural oils in the boards can leach out, and the boards can get used to their environment; they will not adversely weather, warp or deteriorate over this period to the point that they would require sanding.
You will just need to give them a good clean with a deck cleaner such as Cabot's Deck Clean prior to the application of your stain or oil.
Check out How To Clean a Deck and How To Oil Your Deck for some further guidance.
Jacob
