"They can't know that the boy lived to be a man."
Most assumed that the Observers were talking about Olivia, Walter, etc. How do we know WHO they are talking about knowing of Peter growing to be a man ? Maybe they were talking about a specific group of people, or perhaps even Walternate and Bolivia (whom we haven't heard from concerning Peter's "return"). Let's not forget that Peter was originally from the Redverse, not the Blue. He has now "returned" to a Comboverse. He is from neither.
Yet .... it is a Comboverse that HE "created" Afterall, it was he who stepped into the machine, and caused the wormhole bridge between the two to form, combining the two universes. The Observers had a chance to alter the very Comboverse that Peter created but chose not to.
We do not currently see Peter with a bald head and no eyebrows ... however, one could argue that Peter is the Ultimate Observer of this comboverse. He created it, and while in the other two universes (Red and Blue) everything seemed to revolve around him, this new Comboverse with it's new timeline are the result of him. He is still the anomaly, but in a reverse fashion. Previously, the two universes were degrading because of him. Now, they exist because of him. And if he has brought with him time displacement phenomena of some sort ... hmm ... he is now the result of a new "pattern" ... and if anyone remembers Season One's clues, hints, puzzle pieces, etc ... most, if not all, referenced the Fibonacci sequence, golden ratio, Fibonacci spiral, etc, which we saw tonight ...
And lets not forget that Peter walked in and out of those time slips as though he were playing a smooth game of basketball, whilst no one else around him did so in such a fashion. Who else does this ? We know who else does this lol