IMO don't get too wound up about tire age. A new tire that is actually a few months old is just fine. When you store your tires, keep them in a cool, dry place out of direct sunlight, ad they will last a long time.
Also, do not worry about the speed rating. A higher rated tire is not necessarily higher quality, it just has design characteristics intended for higher speeds.
Often, tire designs are compromises. Higher speed tires often have less load rating, because they have thinner sidewalls. These thin sidewalls dissapate the extra heat generated by high speed, but do not have the high strength of heavier tires. Also, higher speed tires often have larger tread blocks. Such blocks are great for dry road grip but do not function as well in slippery conditions.
Tire size...... the theory is to use a narrower tire in winter, for better control in the snow. A 15 inch tire is shorter than a 16 inch, but not necessarily narrower. Also, your suspension, steering, ABS, traction control, transmission, speedometer, etc are all designed for a 16 inch wheel. It's probalby good to leave it as-is.