The people who have perfected kriya yoga have become siddhas, as they have attained the spiritual illumination. You may have heard of Babaji, the ever-youthful saint. He was the master of kriya yoga. Once that spiritual awakening takes place, you have power to control the energy flowing in your body.
You can rejuvenate the dead cells through that prana shakti and not allow the body to decay; it is a natural outcome. After having perfected kriya yoga; the whole body changes, as there is mastery, control and governance over the laws of nature which regulate the normal behaviour of the body. When you arrive at that state of realization, where you are able to use the faculties of your consciousness and shakti, even the body does not know old age or decay. However, the purpose of kriya yoga is not to stop old age and decay of the body. This is a natural outcome of having perfected kriya yoga; you can live a long life.
Kriya yoga has to be understood as a sadhana, not as a practice: “Oh, Swamiji said kriya yoga is wonderful, I can extend my life span, so now every day I practise kriya yoga, and I check myself, ‘Have I aged today? Are there more wrinkles on my face?’” No. That is a futile exercise. If you do that you are not a yogi, you are a narcissist.
A yogi is a person who practises for the sake of perfection and realization, not to check the quality and condition of the body in front of a mirror to be more appealing and attractive to other people.