We also say "you're welcome" when someone says "thank you". I have to admit to saying "no worries" too, but it depends on the context. I'm not sure it's an age thing though, I'm 28 and I know my younger cousins also use the phrases. I think it is more to do with upbringing and use of maners. We are instilling to Harry (and will do with the twins) that he has to say please and thank you and later you're welcome. He can manage "ta" at the moment but he's only 19 months.