I think, in a year Android releases new version and it prohibits to download some apps in the new version. Do we always have to buy and new gadget to ensure that we can download the apps that were designed for the newer version?

New Android OS disallowing you from downloading some app? Do you have some examples?
Usually it is the new Google policy which removed apps from Play Store, not the OS version.

Some apps say that my phone is not available for this version. Some say they only allow Android 6 and above. I have tried some game and finance apps regarding this issue.

android 5 google release 2014 phone company sold year 2015. market share 10%. phone manufacture no longer support this phone.

What phone are you using and what version of Android is it running

Means your phone is android 5 and below which is release in year 2014 and below. It is time to update your phone, it is too old already IMHO.

Some apps not working on old android
But still have some apps not working on newer android

Android 6 is 1 year? Android 6 already more than 4 years . And if you below it its more than 5 years at least .

This post has been edited by xerox8908: Oct 22 2019, 02:40 PM

The app you are looking for may have been removed from playstore or it is hidden from you because your android device is unsupported. An example i s that you cannot run android 4 apps on android 9 and neither can you run android 9 or 8 apps on android 6 and below. New versions of android use new sets of programiing codes. Just update your system or downgrade it depending on the app you need. I wouldnt recommend a downgrade though. It is not worth it.

If your are getting the parse package error then the app was not designed for your operating system. This is what happens when you have the new or old versions of the android operating systems and try to install old or new apps respectively. It wont work. The app developer may have not updated their app or deleted the earlier versions.

the app's minimum android version is actually depend on how or what the dev want the apps to do.

Usually newer android has better library that the dev can use, for example; popup. You can make this feature from scratch and support as many android version as you want, but this might give you a nightmare.

I remember wanted to be a "good dev" and want to support as many version as possible, but end up supporting latest android only because feature that i wanted to use wasnt available in the older android.

To answer your question: If the apps required newer version, and your gadget no longer supported, then yes you need to buy newer gadget.