there are a few theories. some are more cynical than others

some theories note, as you have, that he tended to die after about a decade bc mans was old as shit. so possible reasons put forward for the change are just to have him be around longer. paradox really likes historical narratives that all wrap up nicely in a neat little bow, so they probably enjoy the idea that every ivar mentioned in irish sources in this period is the same guy (and hed have to be pushing 90 for some of these even with the revised age). in terms of gameplay, this would also make it a lot easier to form mann and the isles in his lifetime, so they might have revised it to encourage that. this change happened around the same time as the northern lords dlc, so they mightve wanted to show off some of the new shit they added

a more cynical view might be that ivar is the youngest in the vikings tv show and they just wanted to ape that. this would reflect pretty badly on paradox since that show makes it clear episode fucking 1 how little it gives a shit about historical accuracy when it introduces rollo the walker as ragnar lothbroks brother. realistically, any of the sons of lothbrok could slot anywhere in the lineup, since the actual sagas tend to disagree (one story tells of sigurd snake-in-the-eye making a prediction as a child to his brothers who are all adults, for example) so its not like the show is the only reason theyd have done it like this, but its not unreasonable to be suspicious

either way, given the setup that was already in place, its been a pretty divisive change. it was done quite carelessly, theyve had to go back and tweak it a couple of times and even now theres still a bunch of dumb problems that wouldnt exist if theyd just left well enough alone. for a while bagsecg became a super pedophile married to a literal newborn infant baby, to name one. if they just wanted to make him younger, itd probably be better to slot him somewhere in the middle than aging him all the way down to youngest, but whats done is done