This is correct. All bishops, from auxiliary to the highest cardinal (except for the Pope) must submit their resignations on their 75th birthday. However, the Pope need not accept their resignations, but "will make provision after he has examined all the circumstances." CIC 401.1.
Cardinals are usually allowed to keep their posts well beyond age 75, provided they are in good health. But that is not always the case. A check of [www.catholic-hierarchy.org](http://www.catholic-hierarchy.org/) reveals that there are many voting cardinals who are otherwise retired. Some non-cardinal bishops also keep their posts until the Pope decides on a successor, which can sometimes take awhile. Unusually, though, Archbishop Weakland of Milwaukee and Bishop Sullivan of Richmond had their resignations accepted long before their successors were named, leaving their dioceses vacant. :hmmm: