I think my mini is 5 years old, also. I bought it right around Thanksgiving, but not sure what year. The download took just about an hour (as expected) then the install took about another hour. I was concerned when the status bar said "22 minutes remaining", so I went and did some stuff, came back, and then it was "21 minutes remaining". It stayed like that for a while, then it caught up to itself.

I think I have all the updates. I was thinking of getting a new computer this year. I wonder if this is their way of encouraging me to make a new purchase? I also need a new camera, tv, and some work done on the house