Seems to expose some different assumptions about the purpose of jail time. To me, it's purely protective: you put violent people in jail to physically incapacitate them until the threat of them causing more violence diminishes as they age. That's the reason most Scandinavian countries give sentences of under 10 years for most crimes, and under 20 years even for murder, because the risk of older people committing violent crimes is
very low. So if your goal is to protect society from reoffense, the biggest win is to keep people locked up for their 20s; the 2nd-biggest is people in their 30s; and it gets increasingly pointless beyond that. In fact, giving supervised release instead of jailtime is often warranted, because keeping people tied to their communities / sources of employment is one of the best ways to prevent recidivism, while a few years in jail tends to turn people into hardened criminals (with fewer ties to the non-criminal world, and more ties to criminal gangs).
In this case, 18 years seems pretty clearly excessive from a protective point of view; it's not even clear any jail time at all is warranted, since some sort of probation with supervision of internet use would likely suffice to prevent him from reoffending.
My impression is that many people, especially in the U.S., have a notion of justice that's either more retributive or deterrent based, though. If retributive, that's a pure ethical disagreement. If deterrent, then we might have an empirical disagreement, about which lengths of sentences in fact deter crimes.