At 30, you have finished your third decade, and started your fourth.
.... Wow seriously I know I'm late but I have to respond to this so at 9 years old you are 1 decade old? At 10 you've started your second decade, at 20 you've started your 3rd decade and at 30 your in your fourth decade? 1 decade = 10 years so unless a newborn is 1 decade old your logic fails. Since as you see above by your logic 10 would start her on being 2 decades old.... In reality (feel free to ask a professor or look it up anywhere) the first decade would mean she reached 10 years old, 2nd decade starts at 20, 3rd starts at 30 and 4th starts at 40... Which means saying a 29 year old is almost 4 decades old is most definitely incorrect...
Websters dictionary definition
"a period of 10 years"
As an example they use the following
"The decade of the 1920s runs from January 1, 1920 to December 31, 1929."
The reason this is accurate is 1 day more from December 31 is January 1st which would mean it is no longer 1929 but the year 1930 meaning at 29 ahe is under 3 decades old. Lets count by tens shall we? 10, 20, 30 (so 10 * X = 29? Answer? 2.9 which is ALMOST 3... Not almost 4
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.