Senior ice hockey skates are sized for adult players with a men's shoe size of 7 or above. When dealing with hockey equipment, the word "Senior" is interchangeable with the words "Adult" and "Men's". Most hockey skate manufacturers produce their skates 1.5 sizes larger than U.S. Shoe Size, which means that your skate size will likely be 1.5 sizes down from your U.S. Men's shoe size. For example, if you wear size 9.0 U.S. Men's Shoes, your Bauer hockey skate size will be Senior 7.5. Or if you wear size 11.5 U.S. Men's Shoes, your Bauer hockey skate size will be Senior 10.0.
If you're shopping for a younger player with feet already large enough to wear senior skates, and he is still growing fairly rapidly, it may be wise to consider purchasing skates that are only 1 size down from his U.S. Shoe Size rather than 1.5 sizes down from his U.S. Shoe Size. This will give him some growing room to work with, and until he grows into the skates completely, you can typically fill in the extra space by putting an additional footbed/insole into each of the skates.
If you need help with sizing, please refer to the Hockey Skate Sizing Guide