This was a congressional agreement of 1820 which included the admission of one free and one slave state to maintain the balance of free and slave states in the union.

 The Missouri Compromise

Details:  The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added at the same time to keep the balance of slave and free states equal.  It also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state.  

why what did she do?

answer: 25.8 years old or 25 years old