Is 18 cavities and a needed root canal bad for me if i'm only 19?

Yes: If you really need that amount of treatment at 19, you've ignored you dental health for a long time. You need to eat healthy foods, avoid sugary drinks and snacks, brush and floss, and visit your dentist regularly. These are the last teeth you will have that are not artificial... Try to take care of them.

It's a lot: Yes. In this modern age of fluoridated water, toothpaste and all the information about the importance of brushing and flossing, that is a significant number of cavities to have and also to need a root canal. It's important to get these cavities taken care of and follow the dentist's instructions on how to improve your oral home care to prevent further cavities from occurring.