The line--certainly ULTIMATES--was never meant so much for younger readers as new readers. The idea was that someone who'd be likely to watch a Michael Bay movie could pick any issue up and enjoy it. Ultimate Spidey was therefore targeted to younger readers, but Ultimates was meant more for young adults, I'd say up to 25 or something. Around college age, in any event. Not sure about Ultimate X-Men as, since I hate the X-Men, I never looked at it. In addition, the idea was to put the characters in a version ready-made to be made a film(and the original template for costumes and settings seemed the first two X-Men films), so you could just drop one of these comics on a producer's desk and say, "Let's make this."
This was doomed to failure in many ways, except Ultimate Spidey, which it seems nothing can kill. One of these is that any Marvel comic that lasts a while--especially once you make it part of a "universe"--will by definition become laden with continuity and less accessible to new readers every issue. Another is that what's "cool" and "mainstream" changes and you're forced to change with it, and these haven't. And Michael Bay as a model? When was he EVER cool? That made me sick when I saw it said by writers who should have known better, that comics had to be more like that kind of thing.
And most of all: Marvel found that people like the properties more or less intact a lot better than the version based on what they guess people want. Compare IRON MAN or the first two SPIDER-MAN films--which remained reasonably faithful to their sources--and the FF films, or DAREDEVIL, which seemed if anything more the "Ultimate" versions, and sucked(Ultimate FF has been bad from the start, no matter how much talent has tried with it). Which did you like better? Which did better? The Ultimate line is really just a leftover from before Marvel controlled its own films. Now they don't need it anymore.
So we're left with this, which is to say it seems to be where all the worst ideas are being dumped. And my god, this is fucking ugly and awful.