Short Answer: Yes, it will get lower.
Longer Answer: You should know first that your voice won’t fully mature until you are in your 30’s. Even after you finish adolescence, your voice will continue to change and mature throughout your 20’s and into your 30’s, and these changes are especially noticeable if you have regular vocal training. This happens to both men and women and is completely normal.
Men’s and women’s voices both get lower when they age. As the saying goes, “everything sags”. Skin looses elasticity, and like skin, the vocal folds loose their natural firmness and relax with age. Just look at many popular singers: Billy Joel plays just about everything several keys lower than when he originally wrote them, Elton John too, remarking that his old recordings sound like “he’s on helium”, and Steven Tyler, who no longer emits feral screeches as he did in the 70’s.
That said, there are a couple contributing factors, both good and bad:
Smoking will damage your voice and will degrade it faster over time.
Screaming / straining / whispering will also hurt your voice.
Taking vocal lessons and / or singing with good technique will help you retain your voice well into old age.