Question: Not a very important detail, but how old is Virginia? At first I thought she is 20, because Tony mentions that he raised her for 20 years (when they are arguing after he drops the mirror off the roof of the building). However, at the beginning of the movie, she makes a comment about how it is pathetic that she still lives with her dad. It doesn't seem like such a big deal for a 20-year-old to still live at home.
Answer: Virginia says in the beginning to her grandmother that her mother has been away for 14 years. She also says later that her mother left her when she was 7. That would make her 21. And I believe Tony was just rounding when he said he raised her for 20 years.
Rounding up to 20, from 14 (the years that Virginia's mother has been gone), is a bit of a stretch, though. It's not like rounding up from 18 or 19. He is probably referring to all the years that he has been Virginia's father, with and without her mother around.
Answer: As another comment explains, Virginia is 21. She might feel "pathetic" because she works as a waitress and, apparently, is not taking any college classes, and sees no way to make her goal of owning a restaurant happen. Maybe she feels trapped in her situation.