I started to gain in my thirties, before that I was still slim, yes would gain a few lbs then lose them again easily, then in my mid thirties suddenly my metabolism just slowed so much.. I expect it was a change in my hormones. I gained around 25lb in my mid 30's I lost them on low carb and VLC twice before.. but keep regaining, this is my 3rd time round I am trying to lose the weight.