In El Capitan, I don't have the option under SORT to change to Newest First for Photos.
Under SORT, there is only one option "Keep Sorted by Oldest First".
I tried repair and set "Use as system photo library" with no luck.
Not sure where in Photos you are attempting this. I display mine under Moments then sort into Folders then Albums. You can certainly decide how you want them sorted there.
Use key command Fn + right arrow to quickly get to newest, or instead use the top category 'Photos' which shows them newest first. You scroll up rather than down.
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