Why did Tilly say she hated her and Noah's sixteenth birthday?

Tilly hated their sixteenth birthday because Noah left for the war that night.

But he stayed on till the eve of our sixteenth birthday in September. I hated the birthday we shared from that day for- ward. He went as the other boys did, in the night to spare us good-byes. "Like a thief in the night," Mama said, trying in vain for bitterness to keep her heart from breaking.