I read somewhere that because of its age, the Liaoning is only used as a training aircraft carrier. Is this true ?
No.
As ships, both CV-16 and CV-17 are fitted out as regular combat capable ships.
Being utilized for training/seed roles is a reflection of the PLAN's current peacetime priorities of expanding the human resource base for carrier relevant expertise, it is not a reflection of the ships themselves (either hull age or fit out or whatever).