Answer
We find that although Hindu Dharma has also been called 'Varna-Ashrama-Dharma,' in practice it is largely only the Varna-Dharma that has been followed. The Ashrama system — meaning the first 25 years in Brahmacharya, the second 25 years in Grihastha, the third 25 years in Vanaprastha, and the final 25 years in Sannyasa — is visible here and there, but this system was never universally observed. In the early period, some people adopted it, yet in the lives of the principal inspiring figures of Dharma, its observance is generally not found.
For example, Shri Rama did not observe Vanaprastha or Sannyasa. Shri Krishna did not either. Nor did Arjuna, Bhishma, or Dronacharya.
Therefore, it is clear that in Sanatan Dharma, the system of four Ashramas was never mandatory.
The second point is that the portrayal of the Vanaprastha Ashrama found in the scriptures contains elements that seem like extreme self-mortification. However, the Bhagavad Gita is opposed to bodily torment — 'Natyashnatastu yogo'sti na chaikantamanashnatah. Na chati-svapnashilasya jagrato naiva charjuna.' (Gita 6.16) — meaning, O Arjuna, yoga is not for the one who eats too much, nor for the one who fasts excessively; nor for the one who sleeps too much, nor for the one who keeps vigil without rest — only for one who is moderate in these does yoga become successful.
The Bhagavad Gita does mention the Varna system, but it does not endorse a caste-based, birth-determined Varna system. The Lord linked Varna to qualities, actions, and inherent nature — not to birth: 'Chaturvarnyam maya srishtam guna-karma-vibhagashah. Tasya kartaram api mam viddhy akartaram avyayam.' (Gita 4.13) There is no mention or endorsement in the Gita of the four Ashramas — that is, the life-stages of 25 years each.
The Brahmacharya Ashrama was an excellent arrangement in which only study and preparation for life took place. The Grihastha Ashrama that followed was also appropriate, but it too was never mandatory. Many great souls went directly from Brahmacharya to Sannyasa — such as Adi Shankaracharya. Many ancient Rishis, too, never entered the Grihastha Ashrama.
Yes, it is certainly true that after the age of fifty, a person should turn intensely toward spirituality. However, turning toward spirituality does not necessarily mean taking formal sannyasa. The Lord Himself says in the Gita: 'Anashritah karma-phalam karyam karma karoti yah. Sa sannyasi cha yogi cha na niragnir na chakriyah.' (Gita 6.1) — meaning, one who performs his obligatory duty without attachment to the fruit of action is a true sannyasi and a true yogi — not merely the one who has renounced fire or abandoned all activity.
As age changes throughout life, one's nature also changes. In childhood there is attraction to toys, in youth the mutual attraction between men and women, and in old age a natural turning toward God becomes spontaneous. This is why the Gita says that not only in old age, but throughout one's entire life, attachment to sense objects should gradually diminish and inclination toward God should grow —
'Antakale cha mam eva smaran muktva kalevaram. Yah prayati sa mad-bhavam yati nasty atra samshayah.' (Gita 8.5)
Today, when the average human lifespan is limited to 75–80 years, it has become even more essential that after fifty a person advance vigorously toward God-realization and moksha, withdrawing the mind from sensory objects and proceeding toward the welfare of the self. Hari Sharanam!