This is not an absolute, but in my experience (11 years competitive swimming, follow it still), girls who are amazing as swimmers when 8 are usually not amazing when 18. There are always the athletic freak exceptions (Katie Ledecky and Missy Franklin for example), but I would say that for your average "really good" 8 yo, they are not world beaters as high schoolers.

And yes, swimming will consume your life (and your money at times). It is not a poor man's sport. I was able to play soccer and swim from the time I was 7 until I was 14. But then I had to drop soccer. There just was not time any more.

One thing I will say, if you live in Utah, swim for a club that is not a direct feeder team for a high school team. You do not want the politics of high school swimming being part of your club experience. Just my thoughts on the matter.