What happens when someone takes a life Insurance policy when he don’t smoke but later he starts? Will the policy cover the person if something happens due to smoking?

Just declare to your insurers. They might increase future premiums, but at least you know there won't be issues at time of claim.

There is usually a period of two years (please check individual policy wording), after which the policy becomes incontestable. Claims have to be paid unless there is non payment of premiums or in cases of fraud.