Because teenagers often exclude, harass, and verbally abuse others in the same age group for no real reason. If you're a teen who often ends up on the receiving end of that, then interacting with older people may be the only way you can have a normal conversation. Talking to adults is a last defense against missing a stage of social development and developing depression and social phobia.

That doesn't mean that you are personally required to talk to teens, but cutting them off from adult society completely is cruel.

And it is possible for a teen and an adult to share an interest—baseball, or sewing, or entomology, or whatever. Some people like talking to others who are enthusiastic about the same subject, even if that person is young and "still learning". Some adults even enjoy teaching.