When the characters all bow to him, and he to them, we know that he has finally connected to his past, to his family, and to the island. They bow to him on the word "forever," because that's what art is about, after all, saying something that lasts long after we're gone. Through the painting, Seurat lives "forever," just as the people in the painting do, and just as the modern day George will if he can find his artistic way again. When the figures in the painting sing of strolling "on an ordinary Sunday," two things race to mind: that this is certainly no ordinary Sunday, or that perhaps this is indeed an ordinary Sunday and that there is magic even in the most ordinary of times and places, if you only know how to see it.
But perhaps this is not the real Dot who appears before the 20th century George. Perhaps this is the Dot of the painting, a perfect Dot who lives only in Seurat's "perfect park," a Dot infused with both Seurat's calm and resolve, his clear understanding of art, his deep love for Dot (as observed by the modern George), and also Dot's rich emotions, her love, her enthusiasm for living and experiencing the world.
Certainly there are reasons for thinking this is a different Dot. Her manner is different here, her speech more eloquent, and she has an inner peace and wisdom the real Dot never had. All the good things from both George and Dot have come together in this Perfect Dot; she is the final consummation of their love. She is a combination of a real person (or two real people) and a work of art; perhaps her name has even foreshadowed this moment for us (which might placate the critics who think her name is too obvious), as she has gone from flesh and blood to dots of color and back to flesh again. Like Marie, she is the product of the coupling of George and Dot, and so perhaps she also represents Marie, and the passing to George through the family tree of the accumulated wisdom of this family. This Perfect Dot is, in a way, finishing the job Marie began, setting George back on the right track.
This is certainly not the only reading of this scene, but it's well supported by the text and by the original Broadway performance of the scene (which surely gives us some strong insight into what the show's creators intended). Remember that at the end of the first act, we've clearly left the real park for the "perfect park," as evidenced by the song lyric and the choreographed behavior of the characters as conducted by Seurat.
Here at the end of Act II, the real park no longer exists as it once did (the buildings all disappear), so it's safe to assume that what we see at the end must also be the "perfect park," and within it, the Perfect Dot, the Dot not only created by Seurat in his painting but also the Dot created by modern day George in his head, knowing only as much as Marie told him (particularly in "Children and Art") and he observed in the painting. George's image of Dot is the quiet, peaceful woman in the painting, no expression on her face, no conflict, no sorrow, just a woman looking out peacefully at the water.
There is certainly an argument to be made that Dot matured -- in fact was forced to mature -- when she moved to a new country and raised a daughter. But a basically loveless marriage to Louis and the challenges of adapting to an entirely new culture and society probably wouldn't have given her the profound sense of inner peace she has at the end of the show. And she probably hasn't matured while being stuck in the painting, with no outside forces or events acting on her, with nothing at all to effect change in her.
We have to acknowledge that once Seurat finished the painting, there were two Dots ever after: the real Dot who moved to America and raised Marie, and the Perfect Dot who lived on in the painting and in Seurat's memory. This peaceful, wise Dot at the end is not just an older Dot; it's a special Dot, a different but parallel Dot, one created by Seurat, by Marie, and by George.