Doesn't matter. While that amazon shitshow tells a different story, Gandalf (as Radagast and Saruman) only arrived in the third age, long after the War of the Last Alliance. Gandalf might be infinitely older than Elrond yet wasn't there.
I assume you forgot a "not" after the "but". I just looked it up though, Gandalf left Valinor for Middle Earth around 1000 T.A. I don't know about you, but I'd consider that "long after" the War of the Last Alliance.
I guess it depends. Elrond is around 7.000 years old when having this conversation with Gandalf, so 1.000 years might be like 10 years to a 70-year old. Arwen is less than 3.000 years old, so maybe 1.000 years to her is more like 10 years to someone in their late 20s (and as someone in their early 30s, that's a damn long time)? To Galadriel or Cirdan on the other hand 1.000 years might just be like ”Damn, I slept in again, what age is it?“
You raise a really good point on what the passage of time must feel like to young elves. I'd like to think that elves younger than 1000 years are treated with lots of eye rolls from their elders for brashness, similar to Treebeard telling Merry and Pippin to not be so hasty
It's not about rank or age. It's about who's been present at the last battle of the War of the Last Alliance. Also, at the time of the depicted scene Elrond never was in Valinor, so at this point in time Gandalf definitely easier Elrond's senior by orders of magnitude.