Although it's my personal view, i think it could apply to most women including me. I think that women lie about because of INSECURITY. Unlike men, we are put against double standards as we grow older. One is that, we have to make it in life as an individual, to make sth of ourselves. And the other is, we should always have a man around, then marry one day, have kids, grow old together and bla bla. We have to do both of these simultaneously. But in reality, you only get to choose one, unless you cash out all your luck and land both of them securely. And other than that we all had some imaginations of how our life would be at some point, with dream jobs and prince charming and all. So by the time we get at some age, say 25, (keep in mind our imagination) we must have sth to support ourselves with like a job, but in truth barely enough and not even close to our imagination. Next, We look around and not see the partner we thought we'd have by 25, that's the second blow. And in addition to that, our biological clock is running wild. Not only that, but we are unconsciously programmed to think that we should get married earlier or else men will lose interest in us. Our fading and getting older looks doesn't help too. And so that's the third blow. Not to mention how society sees after some 25/26 age, society sees us a nun or like old creature that can't bare anything after a while. So what do we do to ease the pain from the blows, We LIE. We lie about our age, at least it makes us feel much younger even though it's false. So yeah that's why i think we do it.