Holy shit, I can't believe I didn't realize that before. The Evidence was so completely fucking Obvious.
"Obvious" if you were willing to completely disregard any sense of pattern-adherence. Which is the whole point of Act 6 sure, pattern breaking, but we're only human. I've said it before and I've said it again, Hussie has achieved a remarkable feat in Homestuck - by establishing such a reputation for bending the reader's expectations, he has reached a territory where he can beat you over the head with exactly what's going to happen, and it becomes a twist, because you were expecting it to be anything but the "obvious".
Wait, does that mean that Roxy and Dirk are actually older? Or they were born later, as they would otherwise be almost adults.
Since Hussie can keep pulling cosmic rules out of his ass as long as he wants to, there's nothing to say all players have to be seeded to the world at the same time. John landed on Earth five months after the rest of his friends, it was just extremely convenient that he was still of about the same age and they all played the game on the same day. Guardians/Ancestors land all throughout time; the Handmaid arrived on Alternia centuries after it was destroyed.
There's no reason the players couldn't land considerably closer to or even well after whatever date the Game is supposed to take place. Nothing except anything resembling sense, that is.
Also, that explains the Dersites...
I was going to mention that. Also explains why Dirk has in his apartment all these fancy robots, a levitating monitor, and priceless works of art.