The interviewer is trying to establish how committed you are to staying with the company. The company may invest in you with training so, ensure the interviewer that you're not taking this job until something better comes along.
It's impossible to know where you will be in 5 years but do assure the interviewer that, given all possible circumstances, you could see yourself as a long-term fit for their position.
"Ideally, five years from now, I would love to see myself growing into a more prominent leadership role within your organization. My career interests align very nicely with your company's goals which helps me to see a great long-term fit here."
Entry Level answer example
"I'd like to become a proficient dental assistant and specialize as a hygienist in five years."
Experienced answer example
"Five years from now, I would like to be supervising or managing a team of dental assistants of my own. I feel like I am progressing at a rate that will make this a possibility."