We like to think of the world, and the universe at large as being fully explained and rationalised.
Science has the answer for everything. Bad news, folks. It doesn't. For example, scientists aren't able to explain why the sun's corona is several million degrees hotter than the surface of the actual sun. Nobody knows why 9 out of 10 people in the world are naturally right-handed. In Taos, New Mexico, there has been a humming noise in the air that's gone on for nearly 30 years. Nobody knows why or how it's there.
Similarly, nobody can explain how Paul Rudd is 50 years of age and looks like he hasn't aged a single day past 33 or 34. Logic tells us that the most obvious answer is the correct one. Therefore, we must conclude that Paul Rudd has a picture of himself aging in a cellar somewhere.
We know that he's not a vampire because we've seen him in daylight. We also know that he hasn't had any work done because, well, you'd know. We can also deduce that he is actually human and not a robot with a living human endoskeleton because we talked to him once about his character disappearing up the anus of another fictional character.
Let's also take a look at a production still from 1995's 'Halloween 5: The Curse Of Michael Myers', which was Paul Rudd's first screen role.
Now, let's take a look at another production still, this time from 2018's 'Ant-Man & The Wasp'.
There's a 23-year gap between these two pictures, give or take a couple of months. Now, even accounting for good lighting and favourable angles, it is still virtually impossible to age that well without some kind of mystical or divine intervention.
We can therefore only conclude that Paul Rudd is, in fact, using some kind of special power hitherto unknown to human science that allows him to age this well.
Good for him, and a happy fiftieth to the lucky bastard.