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