Do young, healthy people also get symptoms of gum disease?

Yes, anyone can. If you have teeth, you have the potential to develop gum disease. Various factors influence the risks of gum disease including oral microflora, diet, and oral hygiene. This is why it is important to practice consistent home care and get regular check ups every 6 months by a dentist.
Yes, can happen. There are some that get an aggressive form of the disease when they are teens. No one is immune.
Yes. Periodontal disease is most commonly seen after the age of 30. However, poor oral hygiene can precipitate an outbreak at any age. The disease has been seen in healthy kids as early as 10 years of age.