Is Eveline similar to Polly from "The Boarding House," and why does Joyce make his heroines nineteen?
One similarity you have already noted is that the two young women are about the same age. Both of them must also endure the behavior of a less than perfect parent. Eveline must tolerate an alcoholic, abusive father, and Polly's mother is willing to prostitute her in order to find her a husband. Eveline feels guilt over deserting the family, but Polly doesn't seem to feel any guilt over what her mother does to Bob Doran. Another thing the two young women have in common is the life they are doomed to live. Polly's mother is condemning her daughter to the same kind of life she had with Polly's father. Eveline is willing to sacrifice herself to the same life her mother had to endure.
I can only speculate as to why Joyce chooses his heroines to be around nineteen. This is an age where young women are old enough to marry, but they are still young enough that their parents still hold power over them. It is also an age of innocence for most women during that period of time as they are inexperienced with men and love. Polly allows her mother to use her as her mother sees fit, and Eveline cannot desert the family because of her promise to her mother. Neither young woman has the life skills to deal with their problems and really have no one to advise them.
For more information on the two characters, go to the sites below.