The table shows maximum iOS version that each phone supports. It means if you have older phones then you will not be able to upgrade iOS version after certain time. Usually the gap is 3–5 years.

Now, if you want to use your favorite apps, you should know that most apps out there support up to 4 previous iOS versions. The iOS versions before that are no longer maintained.

So you have have about 7 ~ 9 years before you phone will not be able to run most of your apps due to incompatibility. However, up to 4 years is recommended for optimal performance. After that new iOS versions are going to lag on your phone.

Reference:
iOS version by device