So you think that somebody who commits a crime at 15 and lives to 80, say, should serve 65 years in jail, whereas somebody who commits exactly the same offence at 70 should only serve 10?
If you sentence all killers to life with no possibility of parole, as has been said before you finish up with jails on the American model, where many staff members are terrified to go near the inmates, where people with no possibility of ever getting out become even more feral and killings are the order of the day.
I'm not saying that 15 years is enough for this terror, just that there is a system, designed to ensure that he serves time appropriate to the crime. Many people whose crimes were particularly nasty are still inside years after their minimum period has expired. But we don't hear about them, only the ones that the press deem to be worth a story.