When a lot of the time its almost impossible to maintain post-adolescence really. Not to mention after pregnancy, natural weight fluctuations etc. (I am sure there will be loads of lucky women who DO have one naturally come on this thread to deny this! 
I am asking this because I myself would LOVE a flat tummy....have been Shredding for a few weeks now and am getting some good results, but still seem to have a bit of a "mum tum" (sorry for cringey phrase) ....and look at women who DO have a flat tummy with great envy, but OTOH would not want to kill myself with diet and exercise to achieve one. But have not owned one since I was about 14 sadly :o
And also (sorry if this is partly a thread about a thread) there was a thread the other day that ran and ran about a lady who was going to bind her belly after her baby was born in an attempt to get it back to "sexy" flatness ASAP.