Double knee replacement? Both knees or both bones in one knee? What is the reason you need this?
I was in school with a 67 year old lady who had a total hip and total shoulder. She was already an LPN with an administrative job in LTC.
Total knees are a true miracle for someone with much damage there. They are not, however, as good as a "normal" knee.
There are some limitations to a total knee such as avoiding high impact activity, running, jogging, basketball. Your age should be in your favor.
I have seen many young people with total hips due to RA, Lupus or Sickle Cell Anemia, but I can only think of a few as young as you who had total knees.
Go to a search engine such as www.Google.com
and enter total knee limitations.
Another option would be a single surface replacement. I'd say get a second opinion also.
As far as nursing school, if you want it then I say go for it. NO education is ever a waste.