Tough choices if your DD winds up staying in swimming. The impact on your DD's competitiveness all seems to depend on whether her swimming program is organized by grade through the school and targeted at college or by age through a club system and targeted at the Olympics. USA swimming has the top 10 list for girls by age listed here:

http://www.usaswimming.org/_Rainbow/Documents/0be7cc17-d66c-4cca-a72f-01e24f245c94/girls_scy.pdf

Looking at the 50 yard freestyle, at every age the older girls win and more and more of them can beat the younger girls time. This is true from age 11 through 18.

You can also see the USA national age group records listed here:
http://www.usaswimming.org/DesktopDefault.aspx?TabId=1480&Alias=Rainbow&Lang=en-US

Again the times consistently drop by age with very few exceptions.

Missy Franklin famously continued to swim for her High School after winning at the Olympics and has swum for UC Berkeley in college.

So the Olympic path may be by age, but the school and collegiate path would be by grade.

Hope the additional info helps. The STEM school that will place by test sounds ideal, but it is a tough decision to wait until then for proper placement .