It means what it means. Physically it means 100 years minimum literally since 100 years meant one has completed all the four ashramas of life and engaged in all the four purusharthas, seeing through all the four seasons of life. The figure also means fullness, implying thereby a full life, a complete life, physical, vital, intellectual spiritual.
It was the ancient Indian ideal of leading a full life while engaging with the challenges of life in the world. It is a powerful world affirming statement in the Isha Upanishad on which the greatness of the once mighty Indian civilization was built. It is so different from the modern world negating illusionism and world negating ascetic ideal that has led to the weakening and the consequent downfall of India.
Affectionately,
Alok Da
