First of all, there are important developmental differences between a fifteen-year-old and an eighteen-year-old, as well as differences in their legal rights. An eighteen-year-old, for example, can drop out of school, vote, and join military service, whereas younger teens by law cannot make these choices for themselves. The issue raised in the question is far too broad and complex for a simple yes or no assessment.
What constitutes an important decision? Dropping out of school and getting married are important decisions, but choosing which college to attend and which career to pursue are also important decisions. A fifteen-year-old should not be making those first two decisions independently, but eighteen-year-olds should be able to decide the last two for themselves.
The hardest part of parenting is deciding when to hold on and when to let go. Doing one or the other in every instance is not good parenting. The older children become, the more parents have to let go and give them the freedom to grow and to learn to handle responsibility. There is no magic age, and certainly not 15, when parents can abdicate their own responsibility to guide, and when necessary, to draw the line and hold it.