Weight loss - young vs old, men vs women

I often hear that it's harder for older people to lose weight than it is for younger people, and harder for women than for men.  Is there really any truth to this?  Personally I don't buy it and think they are just excuses, but would be hearing reasons why it might actually be true.

My very unscientific and uneducated opinion is that there really are no differences in difficulty if the different groups are all eating a healthy, balanced diet. 

Yes, older people have a slower metabolism, but that just means that they need fewer calories to get throught the day, right?  So if they eat less they should be able to lose or maintain weight as easily as younger folk.  (FWIW, I started to gain weight in my early 40s so I'm assuming that's when slower metabolism starts kicking in.)

The average man is larger than the average women and also has more muscle, so he'll naturally burn more calories than the average women.  So the average women needs to eat less than the average man.   Simple as that?  I understand that there are some biological reasons that a woman's body tries to maintain a certain amount of fat, but I don't think there is a biological reason that her body tries to maintain too much excess fat.

All this came up since my in-laws were just in town visiting.  They're in their 70s/80s and don't have a clue why they're both gaining weight.  They get in little to no exercise (which is fine, I suppose), but my MIL simply eats more than I do and both make a few (not many) poor diet choices.  They seem to think it's just age and gender....

Brian