Shingles are rated by years of life expectancy, as you know, by your question. As your home was built 15 years ago, the most common rating was twenty years for a spec home. Custom homes tended to be in the thirty year range. Not knowing the area of the country, where the home is located, or the presence of hail damage, the best answer is; How does it look? A certified home inspector can examine it and give you an informed view. Generally, they don’t have a dog in the fight, and will give you a straight answer.