There is definitely a decline in the ability of conversion in aging. However, ubiquinol is oxidized in the stomach to ubiquinone before it is absorbed. So one still has to deal with conversion issues. I suspect that the comparisons in absorption were due to using powdered crystalline ubiquinone compared to liposome, micelle, or nanoparticle ubiquinol (liposome, micellar, and nanoparticle ubiquinone has the same increase in absorption).
The one study comparing ubiquinone to ubiquinol in heart patients has a slightly higher dose of ubiquinol. This alone could be the difference, though there could still be the above affect. I wasn't going to pay for the study, but here is the reference if someone has access:
Supplemental ubiquinol in patients with advanced congestive heart failure.
Langsjoen PH, Langsjoen AM.