Simple question. Do you think people gain the ability to be better at marching with increased age? Is it easier to march better when one is older (20's as opposed to 14)?

It's a matter of your physical capacity. It should be fairly obvious that a fully-developed 20 year old who has reached physical adulthood will be in a better physical capacity (as far as skeletal, muscular, and cardiovascular systems are concerned) than a 14 year old freshman in high school who hasn't made his way through puberty yet.

Probably no difference strictly based on age.
More experience is a different story.

I think you are physicaly and more importantly mentaly able to handle the demands of marching in your twenties than your early teens. There is good reason most corps won't even consider someone under 15...maturity is just not there for most people.
Also if you are nearing the end of physical adulthood your body is more capable of dealing with the stresses being placed upon it.

Mentally, it came easier to me while marching a DCA corps at 31 than at 16-18 in a junior corps. The extra years of musical experience and awareness helped a whole bunch.

Physically... For my first year of drum corps at age 16, I was barely five feet tall and a hundred pounds. Thankfully, an 18" bass wasn't much of a physical challenge. I was much better prepared when I was older.