People can change. However for some crimes society isn't going to take the chance, or they feel there should be a signficant punshment. If this person was a serial killer at 21 I wouldn't let them go at 31 if they'd had a change of heart.

Also some things should and do stick with you to varying degrees. For example a crime in the past will still be in your record. This doesn't mean you shouldn't be able to get a job, but it should be up to your prospective employer and your references to figure out if you have changed.