Well, the easiest way is to create a symbolic link in sample-app that points to the other node_modules.
If you're in windows,
cd sample-app
mklink /J node_modules "../older-app/node_modules"
Not sure about syntax in linux or MacOS, but it will be similar.
I don't really recommend this, though, because though updating or adding packages in one directory will affect both (as both node_modules are really the same), only the package.json file of the directory you are in will be modified.
This implies that, if you share the app with another person or put it in another machine, running npm install won't guarantee that you get the same packages, which can cause errors, so do this on your own risk.
For example, if you go to older-app and update Angular to version 6, sample-app will be modified, too. So you can change the code, developing against Angular 6. However, if you then upload that app to Github or other repository, whoever downloads it, when they run npm install will get Angular 5, which is sure to cause errors.
package.json files must always be in sync with the node_modules directory.