How old was Anne when the book ended?
August 1, 1944 was the date of the last entry in her journal. Three days later (August 4th) was when they were finally captured. Anne was born in June of 1929, so she would have been 15 years old when she wrote her last entry.
She received her diary on her thirteenth birthday. That was then the first year she wrote in it. The diary is sectioned off into the first year and then the second year. At the end of the second year (the ending entry), she was 15. After being captured, she then died a couple of months before she turned 16.