A 17 year old can be charged with a crime as an adult and spend their entire life in jail. At the same time they are still not necessarily treated as adults. Either way if they step out of line they pay a price and the consequences can be steep such as losing the right to vote, not being able to join the military, risk to other jobs on background checks.
Simply put - you are responsible for your actions and words. There might be some forgiveness for youth - but not a free pass.
FWIW to suggest that sending something out something that says KKK in 2010 and trying to back away from it later ….just doesn't wash entirely. At a minimum it was meant to be provocative and at worst it is a mindset that is unacceptable.
The Truth as usual is probably in the middle somewhere and the needle of Hader's thoughts has probably moved over time. Either way he's over the line and claims of being 16 or 17 just isn't an excuse. By that time in life the bounds are pretty clear for this stuff and it was that way in 2010.
If he has changed as he claims - he should simply make his apology and then do the program as quickly & quietly as possible. Hope the mob moves on to the next outrage.