My mom has had diverticulitis since february, 2012. She is 85 and the gostrologist is suggesting surgery on her colon. Is she to old? She is healthy

No. There is no age that is too old. I have done much larger and more serious operations on patients her elder and they have done fine. It all has to do with her overall health status. And .... It depend on how much your mom wants it. Maybe she's tired and doesn't want to deal with it. It's ultimately up to her - but her age does not preclude her from surgery.
Plz See Surgeon. Every surgical decision is made balancing risks v. Benefits; your mom's surgeon will need to assess her risk of recurrent attacks (& severity of prior one's) and balance that w/her risk of major surgery. For what it's worth, it's one's physiologic age that matters, not their chronologic one--a healthy women in her 80's is at much lower risk than a chronically-ill person in their 60's.