The table shows the 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 a 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 The iOS versions before that are no longer maintained.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 about 7 ~ 9 years before your 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.