Ok, good. I highlighted that part in blue...
It depends on who is involved & their motivations for marriage. I think we agree on that much.
People of every century tend to place their own perspective of everything onto everything. There have always been many reasons for marriage in every culture in every century.
Of course. This means in the absence of evidence, we are only speculating.
Interesting. A tradition accepted by the group that did not canonize it's source. Ok, I've seen babies in dirty bath water before, but it makes for a pretty iffy standard of evidence, in my opinion.

This argument always confuses me. Does lack of canonization mean a book is false? The Didache is not canonized, yet it is taken as strong evidence of the beliefs and practices of the early church. The creeds were not made part of the Bible. The letters of Clement and Ignatius were not made part of the Bible (although I believe one or two of Clement's letters were suggested for canonization by some Christians) yet they are considered extremely reliable evidence of what the early church believed and how their early leaders thought. If people believed what Clement and Ignatius and the authors of the Didache and the creeds thought, why didn't they canonize these writings?
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That's standard no matter the age or motivation for marriage.
So he was at least young enough to have had the virilty required to the extent that peers would suspect it. Wasn't he actively working as a carpenter? That isn't exactly a leisurely pursuit.
I think the question, "Is not this the carpenter's son? is not his mother called Mary? and his brethren, James, and Joses, and Simon, and Judas?" (Matthew 13:55) is some evidence that the people of Jesus' hometown in Galilee were not aware of the story in the Protevangelion of James. It could be because the Protevangelion is fictional, or it could be that when Joseph returned to Nazareth with Mary, they tried to live as normal a life as possible and didn't brag about Jesus' divine origin. Obviously, the people of Jesus' hometown were not aware of (or did not believe) the background of his birth recorded in the first chapters of Matthew and Luke, either.
That's where we'll have to agree to disagree. Scriptural evidence suggests Jesus had siblings, not step-siblings.
If they in fact were Joseph's children by a previous marriage, how do you think the scriptures would have referred to them?
People tend to place their own 20th century perspective of age onto Joseph.
Very true. Just as people tend to place their own 20th century perspective on what must be meant by the words "brothers and sisters."
Not that it's an unreasonable conclusion, but is that conjecture or is there a tradition that hints at that?
It is a very old tradition. Did it arise from conjecture based on silence in the gospels or from someone who knew, such as John or James, talking about it before the gospels were even written down? I don't think we have enough evidence to know. But for me, if two traditions are both based on conjecture, there is reason to give more credence to the conjectures of people closer to the time, familiar with the culture, possibly privy to more facts, at least living at the same time as people who likely were in a position to say, "No, that's not true," on the basis of personal knowledge or reliable testimony.
Those would be my reasons for thinking it more likely that Joseph was older and that the brothers and sisters of Jesus were Joseph's children by a previous marriage.