Over the top?

I spend so much time lamenting the weakness of sentencing in this country that I feel I have a right to be angry when somebody is over-sentenced.
A 17 year old had sex with an 11 year old girl. This is utterly wrong from any viewpoint and should normally receive a significant custodial sentence BUT:

1] The offence was unknown to police and only revealed when the man walked into a police station and freely confessed.
2] The offence was 20 years old and the man's behaviour since has been praiseworthy. He has no criminal record.
3] His repentance is genuine as seen by him handing himself in when under no pressure to do so.
4] There is no danger of the offence being repeated.
5] He has no penchant for immature girls.
6] His immaturity at the time has not been fully recognised.

That a custodial sentence had still to be imposed is clear. That he should be added to the sex offenders' register is clear.
That he should receive a sentence of five years and four months after he has lived in a welter of guilt for two decades is over the top.

Where is the perspective? What does the sentence achieve?

I think 12 months in this situation would have been entirely appropriate - and this from a writer who would happily dish out 25 year sentences for this offence under different circumstances.

LINK: http://www.dailymail.co.uk/news/article-1100839/Man-jailed-walking-police-station-confessing-having-sex-11-year-old-girl--20-YEARS-ago.html