Well, what a lot of people are theorizing, including T.J, is that the concept art is only for the intro of the movie, and then after that there will be a (minor) time skip. Similar to the first Incredibles. But I guess we won't know till Friday. I think it would be smarter to have a time skip, because there would be no more character arc to expand on. The first Incredibles already addressed all the problems that the family could have. so if they aged them up a bit, new problems arise, which is smarter for storytelling. But I honestly don't even care that much anymore, I just want to see this movie, I've been waiting (almost literally) my whole life for it.
Hopefully around 4-6 months later (or even just 6 months later like Finding Dory was supposedly going to do till they finally settled with the 1 year later timeline), 1 year later could work as well (which is the most likely case as it worked perfectly for Finding Dory), but anywhere beyond the 2-4 year mark and you run the risk of repeating the same storyline from the first film (also we don't know whether or not they truly updated the models of the family or not, if they did, the concept art would have clearly shown a slightly older family).
Because if the concept art is showing stuff that's supposed to happen a bit later in the film (I just cannot picture Edna appearing in the intro at all this time around (even though she did appear in the intro for the first film as one of the wedding guests) especially during a big battle (the most we'll get a brief phone call), the same goes for the raccoon (who's no doubt going to be an odd yet funny thing to see on the big screen, having it interacting with Jack-Jack would be the most adorable thing since the Baby Dory scenes), the Incredibmoblie (unless it appears after the battle) and the rebuilt Parr household (I can only guess we're getting an updated dinner scene that callbacks to Violet's and Dash's brief fight with one-another), I can understand Frozone appearing to help the Parrs out with the Underminer battle (as he did exactly so for the video game, but it was just him and Bob taking that guy out while Helen, Violet, Dash and Jack-Jack only helped with dealing with the molerat's minions and getting people to safety), but the flying people in the background are the keys to this film's storyline (are they good guys or are they bad guys?) then the story will most likely stick within the timeline that the ending helped set up for this film.
It's pretty obvious that whatever happens in this film will no doubt help set up a hint to the story for a possible third film (and have the kids as adults be the main focus then, having them grow up now after just starting out would be a very bad way of telling the story, not everyone wants a grown-up Parr family yet without enough buildup to that point and the concept art is proving that).