Staying lean in your 20's and 30's is quite simple. A strong diet and weight loss exercise regimen will get the job done when given enough time. As you hit into your 40's and beyond, hormonal changes begin to take place and make weight loss a bit trickier. Here's a video explaining how hormones begin changing at 25 and how this makes it a bit harder to remain lean. She likewise describes 5 Musts for Fat Loss for women over 40.