Diet and environment are two things that can make a difference in life expectancy. Low fat, low sugar diets are best. they need protein, but too much causes health problems. Constant temperature seems to help, as does constant humidity, and calm air. Drafts are not good for meeces. I suspect lighting is important...my meeces are never exposed to direct sunlight.
I eliminated all pet foods containing corn, and bird mix with corn, and anything with corn or corn products as corn can cause cancer in some lines of meeces. There has been a lot of debate about whether corn is carcinogenic or whether poorly bred mousies are more susceptible.
In any case, I stick with whole oats and wheat, millet, corn-free puppy kibble, with lots of variety thrown in like cooked rice, scrambled egg, spinach, tuna, chicken....there are things that are not good for meeces like citrus, tomato...some other things. I suggest browsing through the feeding section in this forum, and you'll see a lot of different ideas on the whole subject.
Clean water, bedding, and accessories almost go with out saying. Pay attention to your mousies eyes, ears, and fur condition. They are good indicators of how well the mousie is doing. Deal with mites or any other pest rapidly and vigorously.
Mousies who are ill should be kept in a separate tank; mousies who continue to be sick need vet care or euthanasia.