Take any pair of old jeans and cut the top of them off. You will get two bags out of each pair. Cut around the zipper, you won't need it.
I used this particular pair as an example to show that you can add material if you need to in order to get the length desired. This pair had already been ripped so I had to add a piece to the bottom and the side to get the size I wanted. This is your bag have fun. If you want a shorter bag, don't add anything!
Since you are using denim, I would advise using a serger if you have one available, if not use a zig zag stitch on your sewing machine to keep it from raveling.
My top piece was a little jagged so I aligned it up with the bottom piece. Trim off the excess.
With right sides together serge both pieces together. If you need to add some on the side do the same thing.
Make sure you take the pins out before you reach them when you are surging!
REady to sew it together.
With right sides together fold in half and align end of waistbands. This will be the top opening. If you want a bigger opening, use a larger pair of jeans. Pin the sides and bottom together. You can sew it straight on the side like a rectangle or flare it out some for a different look.
Serge or sew to finish up.
You can also add a strap out of denim or use trim as I have. Decorate with patches, embroidery, buttons, whatever you desire, make it your own. Perfect for any age!