Will my untreated hyperparathyroidism decrease my life expectancy?

Yes. Population-based studies have demonstrated a 10% decrease in life expectancy of people with untreated hyperparathyroidism as compared to those that have had surgery. While the reason for this is unknown, it is speculated as secondary to higher rates of heart disease.
No. Hyperparathyroidism is almost always due to a benign parathyroid adeoma in one of the four glands tucked within your thyroid gland. This shouldn't affect your lifespan. The level of your plasma or serum calcium may. So if it gets higher than about 11.5, consider getting it localized and removed. As always, check with your pmd or endo.
It may but why? You are 24 years old so what is the reason for you not to treat your Hyperparathyroidism Treatment at your age is Parathyroidectomy and you will have normal life expectancy Consult your Endocrinologist and follow his advice and live a happy normal life.