To begin with, it is necessary to say that there is no one age when men are advised to take pills. There are many factors (physiological and psychological ones). To answer this question, it is necessary to understand the essence of medicine.
Viagra is designed to cure erectile dysfunction or impotence. It means that when a man psychologically wants to have sex, but his erogenous organ can’t get excited, it is time to take Viagra. Every man faces the problem at different ages. For instance, for some men 60 years are the right time, while others start taking pills when they are just 40.
Also, there is a tendency when young guys take the medicine. In their cases, guys are afraid of failing and try to support their organisms by taking pills. Thus, healthy men can also use the drug. In such cases, Viagra serves as psychological support. For example, erectile dysfunction frequently becomes a consequence of stress or nerves. This is a common case for guys who have never had sex before.
To conclude, you see that it is impossible to answer the question. Every organism is unique; it has an individual set of characteristics. So, the topic is rather individual.