ㄴ다더라

I hear they do that

It relays something you heard rather than saw: apparently Koreans get surgery even when they are young, they say friends fall out when they travel abroad together, revenue is said to have risen fifty percent again. What you personally witnessed was the telling, not the fact, which is exactly why it introduces so much rumour and gossip. It ends a sentence casually; for a polite version use 는대요.

Register: casual; rumour and hearsay