I guess many of users that already know and earn bitcoin have already a millionaire today. I don't regret if I know about bitcoin at that time maybe I just ignored it because I'm not serious about earning money at that time.
But I do regret of not believing bitcoin at first, when it was launched at first only few were there who bought it and invested it now they are rich of today and are enjoying there life, but luckily we got into bitcoin at right time and are earning good amount of money from here, making my present good by meeting my financial needs easily and saving some for my future.