I am so sorry Nibbles passed away. 12 years is a very long life. I believe if we use that link Adele gave on this forum, 12 years is well over 100 years old in human age. An average bun life is about 8 years, and the longest I have came across is about 16 years (Emma's bunny).
You should be proud that Nibbles lives to such an old age.
Goofy bunny hugs from my bun Bernie and Minnie