Does your metabolism really slow down as you age?

Data from a study of 6,421 people ranging from 8 days old to 95 years reveal we may shield ourselves from some inconvenient truths in our middle years.

"It's my metabolism"

This is the easy, go-to response when we're trying to figure out why we are slowly but seemingly inevitably gaining weight as we age. 

But how true is this? Is middle-aged spread unavoidable? Or are we telling ourselves little lies so we don’t have to change, because it’s all out of our control?