Answer:
Upon her birth in 1926, she was not the heir to the throne. However, after he grandfather died (George V) in 1936 and her uncle (Edward VIII) abdicated after only 8 months as King she became the heir to the throne as the eldest child of the now King George VI. She became queen in 1952 after her father's death. Her grandmother died just a few weeks before her coronation, aged 85. She celebrates her 60 year reign in 2012.
Her uncle (Edward VIII) died in 1972 aged 77. Her father was 56 when he died. Her mother died in 2002, just weeks after the Queen's younger sister, Princess Margaret.
She is currently the oldest reigning British monarch, aged 85, the next is Queen Victoria who ruled until she was 81.