I own six rabbits and I show them frequently. I never owned a flemish, but i have heard good things about them. Here's the answers to your questions:
* Seven years old is not too young, as long as you supervise her when near the rabbit. I get more concerned when younger kids own tiny rabbits. If your kid is seven, and owns a rabbit that could one day be almost as big as her, I see no problems. That is, as long as she doesn't try to ride it!
*Rabbits can live anywhere between 7 years old, to 15 years old. A Flemish Giant would probably live 8 to 11, maybe 13 with proper care. Of course, this is just an estimate. Your rabbit could live longer, or shorter. I heard of 20 year old rabbits, so the life expectancy is variable.
*A Flemish is not too much bunny, if you are looking at a big rabbit. Keep in mind, the bigger the rabbit, the more cage space (Flemishes need a BIG cage!) you will need, the more it will need to eat, and the bigger the poop!
*As for the average weight of a Flemish, let's just say they easily weigh at least 13 pounds but will most likely exceed that limit
*When asking male or female, both have their pros and cons
Males- PROS: calmer, less likely to bite CONS: messier, and may try to mate with you!
Females- PROS: cleaner, usually don't mate with things CONS: can get bitey during breeding season, more hyper than males
Whether you choose a male or female is your personal preference, but since Flemishes are big rabbits, they are not usually hyper. I would recommend a female for this breed, but there is nothing wrong with choosing a male. Also, Flemishes are nice rabbits and rarely will bite. Male or Female.
*Now, to start with a Flemish can be a lot of work. If you love this breed, get it. However, I think a little bit smaller breed would be easier. I have a New Zealand and they can be a bit shy, but she sits on my lap and gets pet for hours! New Zealands are a little bit smaller than the Flemish, weighing an average weight of 11 or 12 pounds, if they are not obese. Mine only bit me once when she was a baby. The males are extremely sweet and the females are calm. I would recommend this rabbit over a Flemish, but don't be afraid to get a Flemish if you believe that you can handle it! So, consider this info, and if you have any other questions, feel free to ask!
P.S. Is this rabbit going to live outdoors or indoors? Outdoors would probably be easier for you!
Rabbits are great pets and are easy to take care of, so you have made a great choice! Just make sure you know your breed because their personalities differ and some are definitely meaner than others! Good luck!