20 weeks is still pretty young. Most of them will probably start laying in the next two months. While light is the primary determinate for laying in an older flock, a young pullet will often start laying when she is sexually mature, regardless of the length of day. I've had them start laying in late November and early December when they are 24-26 weeks old. I live in PA.
Many folks on here have a mixed-age and mixed-gender flock and made the switch to using grower feed or all-flock feed full time. This is because any chicken that isn't laying, doesn't need the extra calcium in layer feed. To supplement for those who are laying, provide free-choice oyster shells and egg shells (optional). They will become accustomed to eating it as they need it. However, if you already have a bag of layer feed, a short period of unnecessary calcium will not likely cause any harm.