I imagine the ages are just a guideline. All clubs will have different squads and age groups. Our top squad is age 13 to 18, but until quite recently minimum age was 15. We didn’t have enough kids so they brought in younger ones. Squads should be based on ability rather than age generally - although we do have an amazing 11 year old, fastest in the country, but they aren’t in top squad as that would be too much from a friendship group point of view - far too big an age spread in squad.
My DS joined squad at 6 despite minimum age at time being 8. That lower age was then relaxed and others joined at 7. It all depends on what the coach wants and what he is prepared to justify to other parents.
At 8, your DD needs to be really careful not to do too much and burn out or risk injury. Girls generally develop faster than boys in swimming until about 12/13. Girls in our squad usually move up a year before boys of the same age and are quicker. Personally, from loooooong experience poolside, I wouldn’t want to do more than 3 or 4 a week at that age. You need to leave space to step up gradually over the years. If you do 6 or 7 squads aged 8, what do you do at 10 or 12?
We had a swimmer join this year from a very very small club where they swum 3 or 4 age 15. Now 16 they are swimming 5 or 6 with us - a slightly less small club! This year they are multiple county champion and have a realistic shot at British qualifying.
It is great to enjoy it and be successful when young. Mine won county regional and national medals before age 13. Then was overtaken by everyone as he is tiny! His rivals are now 6 foot 3 generally. He is 5 foot 5. He still turns out six times a week and qualifies for everything but is an also ran. He doesn’t care and will never stop. I think if he had over swum age 9 or 10 he wouldn’t by swimming now. Loads quit early teens. We noticed so many missing names at counties this year.
Am rambling, but what I want to say is enjoy it, take it slowly and gradually, and then you will be in it for the long haul. Celebrate the successes - the pbs will tumble like crazy at this age - but don’t wear out the fun too soon and don’t overswim. A good coach will advise you on how many your DD should do at 8. If they push for 5 plus squads, they are not acting in her best interests.
Good luck!