Is sunless tanning can help to decelerates the skin aging aside tanning? How true is it?

As others have said, some sunless tanners have anti-ageing properties. But i would say really its very simple- sunless tanners dont carry the harmful UVA and UVB rays that the sun does which is the main cause of ageing in the first place!

Aging occurs due to a kaleidoscope of factors including internal (your genetic make-up) to external (UV exposure, lifestyle) aggressors. Tanning expedites the aging process by exposing your skin to UV rays and subsequent free radical damage which yields "photodamage" and "aging" in the form of surface discoloration, fine lines and wrinkles.
A sunless approach to tanning will not necessary "decelerate" aging but will help inhibit signs of photodamage - it should be used in conjunction with holistic combination of topical antioxidants (e.g.: vitamins A,C,E, and botanical sources such as green tea, olive oil, etc), and healthy diet rich in aforementioned antioxidants and SPF (important to look for UVA protection).
AHAs are great ingredients but can make the skin more sun sensitive so important to shield with SPF and sun smart behaviour. The exfoliating benefits highlighted by Michelle are great to prep the skin prior to self-tan application.
Hope this helps!