Rats are only good pets for 13 year olds if you have enough money for them, they can be expensive. The main costs for them are the cage and vet bills. They need a very large wire cage, ones like ferret cages are best, and they are prone to illnesses. Tanks are definitely not acceptable because of illness and because rats need to be able to climb, have lots of levels, and they need more space than a tank can provide.
A pair of rats is definitely more work than something like a hamster or gerbil, but compared to a dog or cat they aren't as much work. They need to be played with at least an hour a day, and they cannot go in exercise balls, they need free range time out on the floor in a rat proofed room (if you only got one rat it would need 8+ hours out of the cage). Then the cage needs to be kept clean and they need fresh food and water. That's really it though.
Any animals smells somewhat, but if you keep their cage clean, use the right kind of bedding (paper based is best), feed them correctly (they should have lab blocks) and take care of them, they aren't going to stink up a room. They smell less then most other rodents do. They can be litter trained, but it takes time and patience. It is best to use fleece lining for bedding if you are going to litter train them, it makes it easy to train them and easier to clean.
There is no way to assure they won't get sick, but to keep them as healthy as possible you want them to be in a well ventilated area in a good wire cage, they should get plenty of exercise, give them either harlan teklad lab blocks or a reputable homemade diet supplemented with fresh foods, don't use dusty bedding or pine or cedar, and don't give them unhealthy things to eat like junk food. This will make sure they are as healthy as possible and they will have less of a chance of getting diseases.
Neither male nor female are better than each other. They have different personalities but they are both very social and friendly and make great pets. Some prefer one gender and some prefer the other. Do some more research to decide if you really want rats and then you can pick what gender you want.
It is better to get them from breeders. Pet store rats are often sick because of bad conditions and bad genetics. Plus they have higher chances of getting sick throughout their lives, and they are not socialized at all when you get them. Rats are supposed to be handled from birth, but pet store rats are not handled at all until you get them. So it takes a long time for them to get used to you.
Rats from breeders are well socialized and friendly from the day you take them home, they are much healthier, and you know they have been taken care of well. Make sure to get 2 same sex rats (breeders will not let you only buy one rat).