Almost every kitten acquires roundworms at a very young age (they can become infected via their mother’s milk) and thus treatment needs to start early:
Between 3 and 9 weeks old, treat every 2 weeks.
Between 9 and 24 weeks, treat every 4 weeks.
Small kittens are unlikely to have tapeworms but they can get them from eating prey and treatment can be started from 12 weeks old. In some places, cats will also need to receive treatment to prevent heartworm or lungworm infections – talk to your vet about your kitten’s needs.