I dunno if Terence Tao is as PG as people come, but that might be because I don't know enough about math; his main achievement so far seems to be a joint theorem with someone else, based on the other person's prior work, and I believe I've read that great mathematicians can go downhill as they get older.
[ETA: this may be a misunderstanding, come to think of it; when I say "as PG as people come" I mean something like "so gifted that it doesn't really make sense to think you could reliably define a class of people who were more gifted" - that is, if you had a practical instrument [i.e., not just someone's opinion on greatest achievers] which claimed to identify the most gifted people on the planet and Tao wasn't one of them, I can't imagine what you could do that would convince me your instrument was any use. I don't mean "the most gifted person on the planet bar none" - if you argued that Shelah is more gifted than Tao, for example, I wouldn't want to get into an argument about it.]
Err... splutter... no really, trust me on this, or rather, trust the community of mathematicians :-) You're referring to the Green-Tao theorem, perhaps, but Tao's reputation does not rest on that alone. And some, but by no means all, mathematicians go downhill as they age; the same is true in any walk of life. You're perhaps thinking of the adage that mathematicians often produce their best work before the age of 30, but even that is not always true. Tao certainly shows no sign of going downhill yet; see the work that was awarded last year's Polya Prize for example.
Incidentally I think there's a real danger of rating people based on what we can understand of what they do, rather than on what they do. I don't remember even who was involved let alone what if anything you said on it, but there was a thread recently where I was spluttering too hard even to join in, in which people were opining that physics hadn't seen anyone as clever as Einstein since Einstein. That's IMNSHO total rubbish. Which people end up thought of as geniuses depends on how easy their work is to popularise, as well as on how important it is or how hard it was to do - with odd results, sometimes. (Of course, if the person who claimed this was an active researcher in theoretical physics, I might possibly have to stand corrected, as I'm not one!)
