how old was beth when she died?

I don't remember her age specified when she died. I know that Beth was based on Alcott's own sister Elizabeth, who died at age 23

When Jo takes Beth to the sea, she tries to encourage Beth by saying that 19 is too young to die, implying that this is Beth's age at this point. But then some time passes before Beth dies, and in the next chapter, it says Jo is approaching 25. Since Beth is 2 years younger than Jo, it's likely she is 23 when she dies, which is the same age that Louisa May Alcott's real-life sister (whose name was also Beth) died of the same disease, scarlet fever. So, anywhere from 19 to 23, even though she is portrayed as younger in some movie versions. Margaret O'Brien was only 12 or 13 when she played Beth in the 1949 version.