A five year old cat is middle aged, so you should not see signs of advanced age in her yet, like the cat being less active or starting to sleep more.
If your cat changes behavior, starts to hide more than normal, changes her eating/drinking habits, or changes the use of the litterbox, you need to take her to the vet to find the reason for it.
The normal weight of a healthy cat is about 4,5 kilos, but this will vary by the size of your cat breed; just as it is in people, some cats are naturally skinny and others are not.
If your cat starts to lose weight, you need to take her to a vet to find the reason. One of the most common causes for a cat to lose weight is intestinal parasites/worms, but this is easy to fix with medication that you can give your cat yourself (or let your vet do it).
If your cat lives outside for part of its life you need to deworm her twice a year or more.
Any cat needs to be vaccinated to avoid the most common and often deadly diseases, and if your cat roams free outside this is doubly important.
To avoid your female cat getting ovarian or breast cancer, you should get her spayed. Vets often have weeks where they offer this service at a reduced cost.
Here in the a guide on cathealth.com on checking if your cat is at good weight.
You need to take this with a grain of salt. A cat's body changes depending on how old it is (the skin gets looser on older cats), so to be sure it is best to weigh your cat from time to time.