I think thats one reason. They are stronger when they are older. Another reason is that the roots are much more established and also the trunk is larger. And if its been there since it was a seedling, it would be very used to its surroundings like its climate and especially sun and soil conditions.

Palms become more hardy for the same reason that all plants become more cold hardy over time. They get a more established root system (potentially below the line of frost--which in milder zones, is not very deep). This is why "cold hardy" palms should be planted early in the season--to allow roots to really establish and why it's important to protect for the first few years in colder zones. And I would imagine that as they increase in girth, the root bud is better insulated from the cold. I witnessed this difference in hardiness firsthand this past winter. In fact, despite all the crazy nor'easter, local winter was not too cold (minimum 14.0 F). I had no damage to two larger fortuneis but suffered a lot of damage to a smaller, more recently planted fortunei (which, in fact, will probably recuperate eventually nonetheless). This is one reason I grow in containers until they max out in size. Once I have no container large enough for them, I turn to putting them into the ground. So far, this has worked for fortuneis. This year, I am experimenting with a few others--Sabal mexicana, Butia capitata, Sabal palmetto, Sabal minor, S. louisiana, and Chamaerops. Also finally got to put my Needle palm into the ground. Good luck!

That makes sense as far as the overall survival of the tree, but Im having trouble understanding why leaf burn is lessened the older it gets. Is it because the larger root system allows better water flow to the fronds during cold spells?

That's one reason(and a good one)another is that bigger plants/palms store more carbs and have a higher carb/water ratio.
Higher sugar(carbs)content acts like antifreeze in the leaf tissue lowering the freezing point.
Also resistance and tolerance are more developed in larger/older plants as they are in people,to a certain age.

Thanks guys, Im gettin it now. I think.

Palm leaves get thicker and tougher as the plant matures and is subjected to repeated freezes.