Android does a pretty decent job in making the old apps work but for devs its hard to keep supporting new versions of android. Devs have to use the new apis to be able to release in play store. They have to constantly raise their target SDK version for play store to accept any kind of updates, but to raise sdk version, they have to update the implementation, etc. Then when a new android version release, it have the next sdk version and some apis might be removed and/or deprecated. The newer android can run the apps built for older versions usually fine except if some new android limitation was introduced. But the thing is it needs to be changed when updating target sdk version(play store enforces that you have to make the taget sdk corresponding to quiet recent android version).