Lottie once told Shauna at the end of Case of the Lonely One that she’s a “poor July baby”. Later she said in Author Unknown that she was eight when she first met Shelley. Seeing how she’s logically nine by the end of the year, I add up that way.
She is nineteen, but it hasn’t been as long since wen-tack as you say.
Also I am sorry if my math comes off as overbearing to some of you. This is all just for fun.