Effectively speaking, some automotive parts slowly wear or fail in a way that is predictable. For instance, we know that oil filters collect debris over time, so it's easy to predict when you will need to replace those parts based on how old they are or how many miles you have driven. For these parts, even if they seem to be working okay right now, you can plan on replacing them based on their age or service life.
On the other hand, some components don't fail in a way that lets us predict their useful service life based simply on their age. In other words, we don't typically expect to replace them after a certain number of miles, and it's generally best practice to wait until they've actually failed (meaning: they are no longer operating acceptably, regardless of whether that is a literal complete failure or just a partial failure that leads to bad performance) before replacing them.
From a statistics perspective, this concept is called a memoryless distribution - age doesn't play into the rate at which these parts die. If the part is working fine right now, the chances of it dying today are the same no matter how old it is.
Think of your ignition coils the same way you think of light bulbs in your home. If you had a light bulb that was 5 years old, and working perfectly fine, would you go and replace it right now, just because it's five years old? Probably not, right? You would probably wait until it dies (or at least shows signs that it's dying), and then put a new light bulb in it's place. That brings us back to your actual question: Should you replace a perfectly-functional coil, just because it's old? And the answer is, no: if the coil is not showing any signs of failure (you have no drivability problems, you have no check engine lights, etc), you should feel perfectly comfortable not bothering to replace it right now.
