35 winters
What makes a person an adult? Is it a good job, family, or age lived? How do you know when you've crossed that line after which you're neither old nor young - is this being "adult" or not yet? Constant exclamations - you've finished university and now you're an adult, you'll get a good job and now you're an adult, you'll have a family and children and now you're an adult.
Unfortunately, none of this guarantees "adulthood." As you grow older, you understand more and more - everyone around is a child, some just hide it very well. And perhaps real adulthood comes with the understanding that nothing is eternal.
Time is an amazing thing... While we're young, we throw it around left and right, a couple of years here, a couple of years there. And then you think - God, how much time do I have left... 35 winters. Just think about that.