It's the middle-aged crisis.

You're fifty and a bit, and one day you wake up and realise you will never be the female equivalent of Leonardo da Vinci, and no one will remember who you were after you die. Time is slipping away. Quicker than you ever thought it would. And gravity has overtaken your skin and flesh.

Is there a proverb, or succinct idiomatic phrase that expresses the realisation that time is fleeting by and you still haven't done all the things you hoped to do when you were a young slip of a girl?

I know the idiom it's never too late, what I would like to know if there is an expression that conveys the fear it is too late.