Your first year of life ends when you turn one year old. At one year and one day, you are in your second year of life. Your first decade of life ends when you turn 10, and then you enter your second decade of life. Making some reasonable assumptions, everyone reading this is in their first century of life, and most of us will not see our second.
We see this more often with smaller time frames. If you're in your first semester of college, you haven't finished it yet. She is 29. She is almost three decades old, at which point she will enter her fourth decade, because she'll be done with the first three.