I've run both in 3.5, and I also ran Age of Worms a second time using 5e (mostly during the alpha and beta phases of rules testing).
As noted, Age of Worms is *very* long, and covers pretty a lot of variety in what's essentially 12 linked adventures, most of which cover 2 levels of play. The continuity problems are immaterial for now, you can figure that out as you go along, as there's loads of great advice etc in the old Paizo forums (some is even from me lol). But if it's undead/demons etc you want, over a long adventure path, I'd actually recommend their 3rd Adventure Path, "Save Tide" - I ran that one using 4e rules, using Eberron as the game world, as I thought it was really good also (the last adventures are basically running around the outer planes vs demons etc). Age of Worms and Save Tide are both excellent, but also both very different in tone (one is "stop Kyuss", the other is "stop Demogorgon"; one jumps around parts of central Greywawk the whole time; the other jumps around far-flung places including the Isle of Dread, then goes totally off the material plane for the end-game adventures).
However, I'd NOT recommend you start with a huge adventure path, for a group of younger people who are new to D&D, especially if the stuff they are looking for isn't due for 6-12 months... IMO, you want something that starts off appropriately "small" and manageable, and is in the ballpark of what they will be interested in. You'd run a hige risk of prepping for something that never eventuates.
Shattered Gates of Slaughtergarde was one of the last adventures made for 3.5, which means it probably stands up better converted to 5e rules (more interesting fights, vs decent number of foes not just 4-5 PC's vs 1-2 monsters). It's a decent length, from memory 1-8th level, and interesting enough for mature and experienced players as well as newbies (I had one relative newbie, the rest seasoned vets). You might need to be careful about the exact power level the players have to face, especially as for example Hobgoblins are pretty tough in 5e now (tip: tone down their extra damage to one die not two), but you can do that "on the fly" as you go along and see how good the players are and what they can reasonably handle. The only down-side, from memory, is that like nearly all intro adventures, it starts off with "PC's vs Goblins", then moves to "PC's vs Hobgoblins", and only then starts to broaden out into demonic types of stuff, constructs, and so on. But I think that's the way of things - new players need a simple intro to get them used to the mechanics etc; you can always "re-fluff" the monsters, to make them feel more like they are influenced by demons or whatever...
