How difficult and how long is recovery typically after shoulder replacement in 80 year old?

Not too bad. At 80 years of age, it is important to have a through pre-operative medical evaluation. If you are healthy enough for surgery, the recovery should not be too bad. Regional anesthetics provide good post-operative pain relief, along with ice and pain medication. There is typically 6 weeks of restricted motion before therapy begins. Therapy may last for up to 3 months.
Everyone different. I stopped predicting you does what a long time ago. Everyone is different. Best predictor as stated is your overall health and making sure the patient is in prime shape before proceeding. Most folks by four months are happy they did the operation. Sling in my hands is used for 4 weeks and then therapy for about 6 weeks after this.