Sometimes it's a self-fulfilling prophecy, unfortunately. If you've read Outliers by Gladwell, you realize that by setting that arbitrary age cut-off, we put the 8Us born in the 2nd half of the year at disadvantage. That girl born on Dec 31st might be the worst girl at 8U in 2016, but born a day later, she might be one of the best 8Us in 2017. The older girls at 8U get all of the reps and we get a cumulative affect as they are that much better the next year. Sometimes the nature of the secondary options for playing games aren't great (ball4...ball8...ball12). I think finding ways to get the younger girls into practices at young ages, even if they're not in the 'top12' can mitigate this affect.
I did a back of the napkin study of my league, last season, and there was an almost direct correlation between the average age of an 8U team and their end of season standings. I'm finding that on my team, now, as well. The younger, more experienced, 6U's playing up in 8U are getting passed by the older girls who are playing for the first time.
At this level, IMHO, I beleive age is the biggest factor of W&L's (more so than "talent"). I have younger, more "talented" girls who are getting passed by older girls with less "talent".