If someone says they are "into the 3rd decade of" something (for example, programming experience), I would generally assume that means 30 years or more.
But what you're saying is that it could mean as little as 11 years, e.g., from 2010 to 2021 (yes, 2010 is still part of the '00 decade¹ and 2021 marks the beginning of the '20s, strange as that was to me).
To me, the natural interpretation (at least in contexts where calendar-decades don't have special importance) would be 20+ years. The first decade is the first ten, the second decade is the second ten, and the third decade begins after that.
"into the n-th decade of X" sounds like "n-th decade of X has started but not fully completed yet" meaning "n-1 full decades + something" for me (non-native speaker)
