I was born in Hong Kong and lived there for 13 years, moved to the US and I'm now 40+. I was back in HK last year and although didn't have problems understanding people in Cantonese for the most part, there were situations like calling a restaurant (to make a reservation) where I absolutely could not understand the waitstaff's Cantonese. After 30 years I still consider myself more fluent in Cantonese than English but language and culture had evolved in HK so much that the way younger people talk, especially on the phone where it's harder to hear, is almost completely foreign to me at this point.