"Once you reach 30, you have 'finished' living for 3 decades and you start your fourth" (Yes exactly) however at 29 they are saying she is almost 4 decades old not "Starting her 4th decade" she wouldn't "Almost be" in her 4th decade until she was 39 that is why I said the information in the gane was incorrect
Apologies for reviving an old thread, but your argument here is incorrect. They are not saying that she is nearly 4 decades old. Instead, they are saying she is "nearly in" or in other words about to start her fourth decade (which begins the moment a person hits age 30). If they meant to say that she was almost 40 years old, the sentence would be more along the lines of: "nearly at the end of your fourth decade".
You are focusing too much on what a decade is and missing the context of the entire sentence
A decade is simply a period of 10 years. The word 'decade' itself does not have any inherent positional meaning. Ergo, your understanding of the word is incorrect. The start of a 10-year period is in fact year 0. The 10th year would mark the end of that decade; not the start.
That said, I don’t believe there’s anything more I can add that would change your perspective. So I’ll leave things here