Originally Posted by Triumphman
If that Youth 870 is a 20ga, you can purchase a new Stock for it and install. Only cost ya around $70 to get a Stock Kit from Bass Pro that includes Shoulder and Forearm Stock in Camo. Don't know what the Wood Stocks cost. The 12ga Shoulder Stock will fit a 20ga Receiver with just a simple/easy carving down of a pocket knife or Dremel Tool if you have one at the attachment point. Some of the "Older" 20gauges, I've heard were built using same attachment dimensions as a 12ga, so depending on the age you might not even have to do any modifications.
If your 870 has served you well, this is a very good option. You can also make spacers to lengthen your current stock using wood, plastic jug slices, pieces of tire side wall, and longer screws. Almost any material will work, just find one that looks good to you.
The advantage of making spacers is this. If you hunt in a cold climate, you can remove some of them to make your gun fit better while in your cold weather gear, and put them back in place for warm weather shooting. Cheap solution that my grandfather used many times to fit an older rifle to his taller frame. Place the spacers between the end of the stock and the recoi; pad, and you should be good to go.
OTOH, If you are looking to get a new gun, i would go with the 870. I have had mine for 10 years, and it has worked first time, every time for me. The other advantage to the 870 is that you are already used to the operating system due to owning you youth model. IOW, no need to learn a new gun.
Best of luck, and let us know what you decide.