Stereotyping is a natural heuristic technique our brains use to make sense of the world. More specifically, stereotyping is a type of "anchoring," a cognitive shortcut where people disproportionately rely on the first information input (often visual) they receive in making decisions.
An example of anchoring is the following: If I ask, "Is the price of the washing machine less or more than $900?" then most people would respond somewhere close to $900.
Too often we hear statements like, wow, you're so tall, so do you play basketball? Or she's so much older than her husband, so he's going to cheat on her as soon as her beauty fades. Or you're Asian, so can you help me with my math homework? Or you're a millennial, so how much pot do you smoke?
If people continue to say such statements, then it also reinforces any marginal truth to these stereotypes by acting as a positive feedback loop. Not only are these insidious, but also unstrategic for building relationships. Using these statements reveal narrowmindedness--you've already put the person or group of people in a box as if they have no qualities or skills outside of your initial perception of them.
Because stereotyping is evolutionarily hard-coded into our brains, having stereotypes is natural. We just need to possess the self-awareness and willingness to deliberately challenge them, especially if we're aspiring towards a more egalitarian society.
Every person deserves the chance to define themselves outside of the monolithic cultural, generational, racial, gender, etc. identity into which others may choose to categorize them.