Comment by ghc
At least in my area, cassettes were still popular 30 years ago during the transition to CDs, but by 20 years ago we were in the midst of the transition from CD to iPod. I don't remember seeing new tapes for sale anywhere after about 2000, and we were definitely burning CDs full of MP3s before then instead of making mixtapes. Personally, I bought my last cassette around 1995. Your point still stands, however...I think cutting 10-15y off the estimate would be reasonable.
In my own case CD's, MP3 and cassette use have overlapped. MP3 players were expensive, Discmans were too big to fit in a small pocket. So walkmans were still useful. I used to record tunes and mixes from the radio at the time too as it was super quick to hit record on the ghettoblaster when listening to the radio provided you had a tape ready. So yes I was still using and listening to cassettes in the early 2000's.