DC Girl-I have some of those in my tree, too.Yes, it's amazing how many "premature" first babies lived to grow up, isn't it? Considering that even a modern NICU can lose one born that early...
I've also found that some women had miraculous pregnancies that lasted only six months after the wedding . Sometimes the wedding date was fudged to include the conception of said child.
As for the OP, I'd leave it out.
I also have my eeeevul suspicions when the family records say that there were 3 or 4 kids, born about 2 years apart, and then no more for 10 - 12 years. And then, surprise! -- a "bonus baby," born just about the time that an earlier daughter reaches her mid-teens.
That happened in my family. My little sister is 12 years younger then me and 14 years younger then my older sister and she IS my younger sister. I would be very insulted if someone insinuated that my little sister is really my niece. And I wouldn't think kindly on anyone who has nothing better to do then to count years and cast suspicions around.
I find it offensive as well. Yes, teen pregnancies did happen, but so did secondary infertility, miscarriages, and still births.
I think it is offensive if you are speculating (other than in the privacy of your own head) about people who are around now. I think if you are looking at family history and reading census returns from the 1850s it is a perfectly reasonable, and legitimate speculation.
Illegitimacy was a big deal - it had a huge impact not only on the mother, but also on the child, so hiding it was common. I don't think it is offensive anymore than it is offensive to speculate about (for instance) the story behind any other family relationships you find during your research.
I find it interesting to ponder the stories behind the record. A distant relative of mine did a very extensive family tree and in one case he commented on a great, great uncle of ours who died in an accident in middle age. The census record shows two more offspring born after he died, the first legitimately within a few months, the second two years later....our historian comments that "it is not known how he fathered a child two years after his death" LOL
I noticed the child had two older teen sisters, and in a later census, she is found living with a sister and BIL, who named her on the census as his "niece", not his "sister in law", which confirmed my suspicions that she was not the child of the great uncle's wife.