Miyuki chan's age

At first I belived that Miyuki was 17, but then I realized that Kagami and Tsukasa were born in July while Miyuki was born in October. There is a three month difference between their months of birth. Miyuki I believe is 16 years old at the oldest. I figured this out by looking at the lucky star birthdays and when they appeared in the series. Konata turned 18 in the middle of the series. She was 17 at the beginning, this makes her 17-18 throughout the series. Kagami and Tsukasa had their 17 birthday somewhere in the beginning of the series, making them 16-17. Taking the three month difference into account and the fact that the girls' ages are about a year apart Miyuki must be somewhere around 15-16 years old. Mostly 16 throughout the series. Making her older than Yutaka but the youngest out of her small, close group of friends.