Alpacas live between 15 and 20 years. As alpaca breeding only started in this country around 20 years there have only recently been alpacas dieing of old age. Older alpacas can sometimes feel the cold more than younger ones and they may need some extra feed. Females can often breed until their late years, although it may be worth considering only breeding older females every other year to allow them to fully recover from nursing a cria.
If an alpaca dies unexpectedly it is worth getting a post mortem carried out. Your vet should help with this.