Clearly they are reporting the findings, but what would interest me is exploring the benefits to children of older fathers and establishing at which point the competing forces balance.

The obvious factor is increased wealth - this tends to be backloaded, thus there is a benefit from greater available income to spend on a child, whether this is in the form of better nutrition, better education, greater parental availability (not universally going to increase with age, but the option is more easily there for a well off father vs a father just making ends meet)

Other factors may be less clear, less universal and probably not so widely agreed: things such as an older father potentially having a child with fewer siblings or being more settled (in a maturity sense).

And finally, it would be interesting to look at this in a two generation scale: what of children born to young fathers but where those fathers themselves had older fathers? Is the effect passed on or does it get reset?