I'm 51 and have been on PB since March 2010, with a couple of months where I was only doing it 80%, now it's more like 90%. Ideally I wanted to lose 20 lbs to be back to my "fighting weight." I lost 6 lbs in the first 3 months, then completely stopped losing for the next 4 months. Frustration caused me to slack off in a "why bother?" mindset so I started eating more carbs. That caused me to regain 3 of the 6 lbs I lost. In November I recommitted to a ~95% adherence to PB, resulting in a loss of 1.5 lbs in the month of November. As you can see, losing weight on PB has been very slow and difficult for me in spite of exercise and mostly-adhering to the plan. In my case it may be a hormone imbalance causing me to hold on to this last 15 lbs I need to lose. I am planning to get a full hormone panel done at the naturopath's next week. A couple months ago a thyroid test showed TSH of 2.7 which my doctor said was "normal" so this time I'll be testing estrogen, progesterone, testosterone, DHEA, and cortisol.
I'm a quitter...but I'm back now.