Can I pin a custom startup file permanently? I would like to create a start-up file that does not move from the menu, and disappear after I select and load a few different project files.

I previously set up files but they keep moving out of the menu start-ups when I choose other file names.  How do I pin this permanently to the menu, and can I add more than the 5 or so available without opening up extra file windows?  Thanks.

To be more clear as some misunderstood, I am looking to PIN files permanently to the recent files menu on start-up, permanently, as more than one start-up file.  The below solution only changes the default start-up = 1, not many as preferred.  Every time I use a file that I set up as a template, I have to save the file as a new name for the project, as I should, but the template is then replaced in the "recent-files start-up menu", so how can I pin them or add more default settings for extra start-up files, just to be clearer.  

Or will I have to have a separate folder full of "templates" and navigate to it on start-up, if pinning is not an option? Can I not just alter the default 5 menu options such as "2d", "3d", "video sequencer" on the left? Thanks.
This is actually really easy to do: go to the File tab, and scroll down until you see Save start-up file. Click on this, and it will make the current Blender setup the default setup. This can also be done by pressing "CTRL-U" on the keyboard.