You can not determine age by heat cycles. All female's start at different ages and then come into heat in different cycles. For instance, my Dixie had her first heat cycle at 9 months and she comes regularly every 6 months. Her mother, Dancer, had her first heat at 11 months and comes in every 8 months. I have a friend who has a 17 month old female golden and she hasn't come into season yet! This is not the norm but does happen. The only thing guarenteed with a heat cycle is that it last an average of 21 days, seven days going in, seven days in, and seven days going out. And then again, not all females drip blood. It depends on how heavy the flow. Some you don't even know they are in heat. This is usually the smaller breeds. Labs usually flow heavy because they are a larger breed. Dixie drips occassionally but for the most part keeps herself really clean. I use pads at night on her.

Did you have the lab when she had puppy teeth? This is usually a good way to determine age. Puppy teeth normally fall out around age 4 months. Other than that, I don't know how to determine your dogs age. Maybe someone else on the forum does.