Start her with 10 or 12 years. I first played Fallout with 10, but I only fully enjoyed it when:
a) I learned enough english to understand the whole game. That happened when I was 13.
b) I was mature enough - I think I was mature enough with 14 or 15 years. Mature age is pretty hard to determinate. There are people who are mature enough with 10, and others who still are immature with 20. I think 15 years is the perfect age to meet Fallout. You could introduce her to, say, Arcanum, when she's at 10 or 12. Arcanum is mature, but lighter than Fallout.
c) I ceased being scared of gore and started to like the gore in my Fallout games. 15 is enough to like the gore.
d) I understood at least some pop culture reference LOL.
I think Fallout, compared to Fallout 2, is pretty light. You could introduce her to Fallout at 12, or if we're more cautious here, at 13. Fallout 2 should be only played with 15 years or above.
Fallout 2 is a lot more mature and more heavy. I always thought that Fallout 1 should have got a teen rather than a mature rate, while Fallout 2, for being heavier, would get a mature rate.