I don't think age by itself should be a problem. I am an 81 YO and had my TKR at 80 without problems. I would make sure the surgeon uses regional anesthesia, not general and is an experienced revision surgeon. For example, my incredible surgeon spends about 20% of his time doing revisions of other surgeons' work.

If the 80 year old has other health issues, such as a heart condition, that could change the outlook.

Just make the revision surgeon search a thorough one. Whether an original TKR, a partial, or a revision, finding an extraordinary surgeon can produce great results.