And 15 yo girls who married were typically marrying men who were considerably older.
The median age of first marriage for females in the U.S. from 1890 on hit a low of 20.3 years in the 1950's and 1960's, and for males at 22.8 around the same time. That's the *low*; the median age in 1890 was 26.1 for males and 22.0 for females.
You're certainly right, however, that a 15 yo would have been expected to work, often very hard. He or she would have been working for his or her parents; that's how a lot of work got done, and still gets done, on farms.
My grandmother married “late” in 1936 at the age of 25. For all the good it did her! (’nuther story)