iphone5s

That is a very old phone which can only be updated to iOS 12.x.x.. Only you can decide when to replace it. As long as it serves its purpose, you can certainly still use it. But if you want all the newest features and all the security updates, you might choose to upgrade it.

That depends on whether it still does what you want/need, and how much longer your cellular carrier will provide service to that device.

In the US, it would be completely unusable as a phone by the end of the year.