Indians do not celebrate birthdays. Pious people go to the temple and conduct a special archana on their birthdays, not according to the English calendar, but according to the star of their birth. Celebration of birthdays has become universal as part of the several new-fangled Western habits that are becoming popular. What was common in Indian households was the celebration of the first birth anniversary of the child and the sixtieth birthday which would be a mini wedding festival.
These are the celebrations of the birth of the body. The birthday I speak of is the birth of the SOUL at a higher level than this embodiment will permit. In our tradition, the Soul is not born except when it leaves the body to be reborn elsewhere. For one who takes to spiritual initiation, initiation is considered as the second birth, the first being the physical birth. Even this second birth will set the man on his search of the soul.
Initiation as well as realisation are supposed to give a further lease of life equal to what he has so far lived. In a conscious person, conscious of his soul, on the birthday the Soul seeks a rebirth, a birth into higher consciousness. Celebration of his birthday is the celebration of this higher birth. It is not necessarily once in twelve months. It may be earlier depending upon his own awakening in the Soul. For those who do yoga, it is a yogic possibility that they can be reborn everyday. Others find such men wiser than their years. One Muni who saw Sri Aurobindo called Him Puradhana Purusha and explained that in Him he found the wisdom of 500 years. Maybe He was reborn in His Soul 500 times in this birth.