You must also consider the "aid" factor.
When you join a new server, there's a chance there's several people joining at the same time, and there is the hope of having a party with all of them and having a blast at first.
When you join an old server, there's a chance that the server is already flooded with its own social circles, and people are unwilling to spend their time helping an "unknown" person.
MouRO is pretty different, though... because it has been up for the longest time, it just doesn't has a "rush of people" joining at the same time to party up, but the server is designed around partying, which makes it easier for veterans to help lower level people up to their level to party together. So... some of the older players actually had it harder as they had less help back when there were less people around, and some of the newer people actually receive a bit of too much help (items, extra-equips, party-based exp boosts and so on).
To illustrate, the first person who ever reached the max level (150) was Bulbasteve, and it took him a few months to get there playing from time to time (mostly because there was no one high level enough to party with, and he gained most of those levels with lower level parties where he just played support - he is a priest). Some months later, when we got a bunch of people doing high level parties, lv150 chars start showing up everywhere. This is true to the point where the "char Top 50" ranking became useless because it was filled with lv150 chars :v
Now the problem is that sometimes new people are helped too much, to the point that in a single day they have level 100+ and a bunch of stuff, but they were helped so fast they didn't really learned what they are doing or how to effectively play around, compared to those that slowly climbed their levels xP
Anyway... enough history, that kinda should illustrate how the server's age can play a factor in the party-making process for new people, and that can be a big deciding point for some (what is a MMORPG if you are not going to be partying with others?)