I consider behavior, not age, in deciding how I will respond to a person. I think it's a real shock to a parent when the child who has deified them in the past changes and now looks upon them with amusement, hate and loathing. So begins the years'-long battle until the kids are gone. This is a learning experience for both parties that neither has prepared for. To be lied to, ignored and embarrassed by your teen children naturally gets carried over into how one sees all teens.
Adults still see themselves in their mind's eye as young.
I feel young, think young, I AM young. I know how young is supposed to be enjoyed. One of the most remarkable and irritating acts is when a teen wakes up on their 18th birthday and has suddenly absorbed massive amounts of wisdom and knowledge from their pillow overnight, and refuses any suggestions or ideas or someone else's take on the realities of the world - yet they still require regular infusions of cash. As you can tell, I still need closure on some things. This type of behavior takes years to learn.