Depression is widespread among elderly, and usually it is undiagnosed and treated. generally, SSRIs such as Fluoxetine, Citalopram and Escitalopram tend to be the most commonly prescribed antidepressants for patient with major depression. SSRIs should be prescribed for this patient because this drug has reduced risk of pharmacokinetic interactions. Also, safety and side effect profiles of this drug makes one of the best choices among depression drugs. Further more, SSRIs are generally are better tolerated than other drugs that treat depression.  (Birrer et al., 2004). 
It is very important you let the patient know that antidepressants drugs such as SSRIs start to work within 2 weeks of when you start taking them. But it usually takes at least 4 to 6 weeks before you get the full effect, and might take as long as 12 weeks. (Birrer et al., 2004). Also, you might have minor side effect such as feeling anxious, feeling tired, dry mouth, problem with sex, nausea, diarrhea, or constipation and if that happens continue taking the drugs.
Some of the positive changes that can be seen on patient taking antidepressants medicating are improved concentration and improved though process. Patient will see improvement on their sleep pattern. But, if patient doesnt see any improvement taking this drug, the prescriber should try increase the dose or switch different  type of antidepressant. ( Shelton et al., 2001) 
Reference 
Birrer, B., & Vemuri., P. ( 2004, May) Depression in Later Life: A Diagnostic and