"Kids today. They don't know they've been born."
What does the idiom 'Not know you are born' mean? Discover the definition of 'Not know you are born' in our extensive dictionary of English idioms and idiomatic expressions.
www.usingenglish.com
Using English claims it is international usage. However, Google Ngrams suggests it is primarily British English. Earliest written record seems to be about 100 years ago. To me, it feels like its more Midlands/Northern British than southern.
Most reference websites seem to suggest it means something along the lines of "you have a good life and don't realise it". It's something like "Oh my sweet summer child" in that regard, except a bit more disparaging, because it's normally used to refer to someone in the third person, rather than talking to someone in the second person.