photo library

It depends on what view you're in. Here's a graphical representation of the different views in a Photos library and the sorting options for each:

You can also create a Smart Abum with the criteria "File name contains .". That's a period.

Then you can sort them with the oldest on top.

Latest photos are always at the bottom of the list in the all photos view - and that cannot be changed.

However photos will always open to the last postion you were viewing. If your view (when you closed photos) was at the bottom of the all photos veiew (where new photos are) then that is where it will be when you re-open.

The library normally opens on the last album/photo you were viewing when you closed the library.

Is it the sort order of the library you want to change?