I think for most, what comes naturally at an earlier age becomes more difficult as you age. It is very EASY to add weight and it has become somewhat acceptable. Take a look at the latest BMI charts and you can see where you fall.
It took a lot of work to get my 6' frame in the "healthy" region of the charts. I'm at 175. I actually could get all the way down to 140 and still be considered ok in the BMI charts. A lot of observers look at these charts and just think they are ridiculous. Being slightly to mildly overweight has become the norm for most of us.