By 1830, the throne had passed to George III's third eldest son, William IV. However William IV had no legitimate children. The heiress presumptive to the throne was his niece, Princess Victoria of Kent, who was only twelve.
As Victoria's father was dead, the Act placed any potential regency caused by the King's death before Victoria had reached 18, in her mother, The Duchess of Kent. However if Queen Adelaide gave birth to a child, that child was to become king or queen instead of Victoria, and Adelaide would become regent. If such a birth occurred after the King's death, his child was to succeed Victoria as king or queen.
Victoria became queen aged 18.