Welcome!
It is true that scores tend to be more stable beginning around age 8 or 9, but that doesn't invalidate your DC's current scores, or imply that decision-making based on them will necessarily be faulty. Even if were to be found that her scores were inflated due to environmental or personal factors that will lessen in impact later in development, she is currently performing at a sufficiently advanced level that a discussion about grade or subject acceleration is worth having.
You will likely find when others chime in that there are many factors that go into an early entry decision, among which are the global development of the child (including social skills, executive functions, and fine-motor/hand-writing skills), the academic/non-academic nature of the receiving program, the length of the school day, and the availability of enrichment and afterschooling resources outside of the school day. For some, early entry will hold off the need for further major changes in educational plans for a little while, while for others, the need for sufficient play and exploration time is developmentally more important than an advanced academic setting. Also, those who score in this cognitive range generally will need many additional modifications to their educational programming over the years, so this won't be either the last time you will have to go through this decision-making process, or the last chance you will have to alter the trajectory.
IOW, this one decision is not going to be the be-all end-all of managing your DC's educational experience. Be flexible, and trust your instincts. Don't be afraid to change course if you have to. If you try something, and it doesn't work out, it's not a failure, just a learning experience--or experimental data, if you like. Breathe!
Oh, and she appears to have qualifying scores for DYS, so you may wish to apply for that as well.
