^^^^ really is the key that for a 3 piece it needs to be perfectly tailored, so whole package is presentable and in harmony. But how many young guys can afford a good bespoke suit?
With the above example, Alan was wearing an Italian bespoke 3 piece, with good body shape anyone can look really good at it. Doesn't need to be old or young, it is just ugly when someone with a belly tries to wear a waistcoat.
And thus, the reason I've never worn a 3-pc suit -- though you can take out "bespoke" if you like. That said, of course, you can still look a little out of place and uncomfortable in an impeccably tailored suit that costs more than my... well, I don't even own a car. My girlfriend's car, certainly.
Honestly, I think three-piece suits look fine on heavier guys -- if the fit is right.