I didn't understand the final scene. Somebody could give me a glimpse?

I thought Jonah and his wife had a daughter.

Your interpretation is as good as any though because that last scene was bewildering, to say the least.

And all the pieces matter (The Wire)

There are other dream sequences in the movie that kind of show the angle the last scene is being taken, where there's odd or nonsensical elements.

In all the dreams (including the mother's), they're set up like real dreams -- a kernel of truth in an overall theme with randomly strange or disconnected facts/happenings. At least, that's how my dreams are.

I think "the old mean) baby" represents inner struggle in more than one way. First, the mother was always gone filming war scenes that involved children. The younger son states that he always thought she should just bring the babies home, ie: solve the problem at home instead of being a passive documenter of the conflict elsewhere. The mother doesn't do anything to directly help the people in the war, neither does she resolve any problems at home.

Obviously this is a nod toward the older brother, who actually has brought a baby home. He is the one who is struggling with issues about having a family; this is hinted at by an earlier scene where the younger son writes that his older brother looks like/reminds him of his mother. I'm not sure, but I think this implies the younger brother knows that even though everyone was worried about him dealing with his mom's death, it's actually the older 'successful' one who is having a hard time dealing with it and the similarities he's feeling in his own life. He says "the (old man) baby" is his brother's.

I think it's implied that the younger son had been coping with things his own way until the retrospective/show was being put together and people worried how he'd react finding out his mother's death wasn't an accident. It is kind of implied that he knew, in the scene where it shows the dad driving at night, and from the back seat he sees his dad nodding off and almost driving into the path of a truck, and his mother in the passenger seat did nothing to wake him or prevent it.

Just my thoughts. I don't think the old man has any significance, other than to perhaps make a point about old problems/new problems being the same, or just to make it more abstract, like a real dream would be. If anyone has other ideas about what the old man might mean, I'd like to hear it.