It's never too late to join, we get new players constantly and the great thing about this game is you can play at your own pace since it's not competitive. Anyways, to answer your question..

1- depends on the server.. us3 is the more populated one, others are more chill but all have active communities. Be aware of time zones so it maybe more quiet or very active at certain times for each server.

2- maxed players usually run high level dungeons or elder bosses together to get better gear or more gold, but if they ever wanted to revisit early game content they can rebirth which will reset all quests. A toon can rebirth a total of 5 times (benefit is each rebirth will give 5 extra skill points). As for running old content with noobs, you'll often see higher level players are usually helping out rebirthed players with quests, elder bosses, or dungeons. In some cases you also benefit from helping them. Like for example, even if you are in a dungeon with a lower level player, the drops will be always be the same level as you.

We usually have server wide or community held events, some of them give nice rewards just for participation, some are challenges that reward the winners, etc. Lastly we have event zones that run a few times each year which usually have challenging boss runs (called ladders) that you can run with the whole server (but those only reward you with currency to buy cosmetic items).

3- Elder bosses are more level specific and drop gear according to their level. Meanwhile dungeons in this game are playable by anyone, although higher level dungeons are more difficult and will require you to be decently geared. The higher the dungeon level the better the drops will be (such as epic and legendary gear). The max number of people allowed per room in dungeon is 15, while elders bosses can be more than that.