Now that smoke alarms are so common, many fires take place in homes that have them because the batteries are dead.
I recently went to a seminar on smoke alarms. One of the nice investments is to have alarms that have 10 year batteries.
You don't have to climb to replace the batteries so they are safer that way for elderly and when the battery runs out 10 years from now, you throw them and get a new one. While they cost more initially, most of them save you substantial money over paying for the batteries over 10 years.