Hi, sorry for late replay missed this,
Sorry for your loss, but she sounds as if she had a full long and happy life.
I knew a very old boxer, full of lumps and bumps, had cateracts but still went on a forrest walk, not far, and he plodded along at his own pace. I dont know how old he was exactly maybe around 14 / 15 as I recall chatting to the owner about what an impressive age it was, I think at a guess 12 would be a good ave age for a boxer.
Good luck with finding a pup.