What does the countdown to adulthood mean?

The countdown to adulthood means that we will be 18 years old soon, and there are only a few days left, so we have already started the countdown. Adulthood means reaching the age of adulthood and reaching maturity. Countdown refers to calculating the time from a certain point in the future to the present, which is used to show how much time is left before a certain deadline (mostly, time is getting less and less, and it is getting more and more urgent).