Marissa's Age

Evidence she is a teenager.

 * She attends high school

Evidence she is in her 20s.

 * Classmate Mr. Vibe is 23 years old in 2018, which could make Marissa about 23 years old in 2018.
 * Marissa's first known 17th birthday party was in 2013, which would make her about 22 years old in 2018.

Evidence she is in her 30s / 40s

 * She has surprising knowledge of 80s music and movies.