I'd think the opposite. Since she is used to having to work hard from a young age, she will be more successful in life.
Others who coasted through—whom were used to everything being relatively easier—will not have the experience she has already accrued when faced with a harder challenge.
Not sure that makes sense. She potentially experiences 12 years of increased stress compared to her peers. I'm not implying that later in life, she will have better tools for work, but that her body would have spent most of her formative years under more stress.
This would then reflect in a shorter lifespan.
Note however that this concept she would gain the tools for hard working in itself is flawed. Studies have shown that those born in the Summer months have statistically lower exam scores.
This is very hard to measure as you would be looking at teaching practices in the 1920-1940s to determine the effects on life expectancy. I think potentially, nationally, this would be possible as most nations have been quite consistent on their school starting times.
I do think having a breakdown by month, not just quarter could be even more insightful.
Malcom Gladwell's Outsiders book basically covers this topic. Usually the ones who do best are the ones with an early head start.
One example that I remember is that oldest kids in class are usually the biggest, that (generally) makes them better at sport early in life. That leads to extra coaching for sports teams, more practice and the effect snowballs.
I don't have a link, but there was a comparative study done on children in Norway during the debate on lowering school starting age (it was 7 years when I was at school - it's 6 years now, and there's some pressure to lower it to 5).
They compared salaries and other factors based on age at school start and found basically no statistically significant differences later in life between those who had been oldest and youngest when starting school.
They did not look at things like health or stress levels, though.
Others who coasted through—whom were used to everything being relatively easier—will not have the experience she has already accrued when faced with a harder challenge.