Charaka Samhita on the Importance of Rasayanas:

Rasayanas replenish the vital fluids of our body, make us look younger and increase our life span.

Good memory and intelligence, long life, health, youthful life, good complexion and voice, broad mindedness, increase of strength of the body, and sense organs; success in speech, sexual prowess and brilliance are obtained by rejuvenation. 

Rejuvenation therapy administered without purifying the body does not help, just like colouring a dirty cloth does not shine.

Rasayanas are of two kinds : Kutipraveshika and vatatapika where the first one is done in isolation and the second one by taking medicinal formulations.

A suitable rasayana agent is selected depending upon a person's age, constitution, adaptability, the condition of sense organs and digestive capacity.