When I started working for the HETE project as a young impressionable grad student, another scientist on the team gave me the rundown. There were "adults" and there were "kids." The "adults" on our project were a group of 6-7 principal investigators, senior scientists and full professors who were generally responsible for the broad scientific direction of the mission, press-releases, grant-writing, political arm-twisting, etc. Many of the adults maintained only a general understanding of the underlying working of the satellite and the software. The term "kids" wasn't limited to just mean grad and undergrad students, but also the group of non-professor-level Ph.D. scientists who actually ran the mission and knew what was going on day to day. When we went out for team dinners, the adults and the kids often sat at different tables and had entirely different types of conversations. Kinda like Thanksgiving with my family, in a way.
Now, having participated in two largish scientific collaborations I can say with certainty that the most crucial member of any team is the person (or sometimes, persons) you might describe as the "oldest kid." I think any good collaboration must have at least one of these. As an entering student, they are absolutely the person you go to when you want to know something and really understand the details of it. They tend to have an encyclopedic knowledge of who is an expert on 'x' issue, which students are working on 'y' calculation, and the best way to extract 'z' data product. They are traffic-directors extraordinaire. And most importantly, they communicate the nitty-gritty details upward to the adults and make sure they don't make dumb decisions for lack of understanding the facts on the ground.
They often don't publish a lot of papers or get a ton of recognition. Some may work an entire scientific career without truly joining the ranks of the "adults." But I'm convinced that without these oldest kids, the entire scientific enterprise would grind to a halt. I can certainly attest to how absolutely essential many of these people have been to me during my time in grad school. So here's to you, oldest-kid researchers! Huzzah!