I may not be the best person to offer advice, not coming up in this situation before but a few thoughts that cross my mind when reading about your posts.
As other people have mentioned your age is something that counts in your favour especially work wise. As much as it may feel like it will hold you back which is completely understandable, in 10 years time all it will show is that you made a foolish mistake and have learned from it, the truth behind it or not. the issue with cases such as this will be not that it happened but whether it was a continued issue within your life, and i get the impression you really arent going to put yourself in that position again so i would worry about that a little bit less.
From what you have said about the 'crime', its a result of the touchy racism term that this country holds. From what i gather you had a fight with someone, over what doesnt matter. They have used their skin colour afterwards to get back at you and as a result you are here. this isn't as bad as it feels right now mate, time will show that to you. If you had targeted this lad and really injured him you would be behind bars in a youth prison or whatever the rules are and the crime would reflect worse upon yourself. No jail would suggest to me exactly what you say and therefore the lack of prison time again will be used as a strength of character rather than motive of intent in the eyes of future employees. Especially at your gae (sorry for saying that again!).
As for your family and friends, i can fully understand what you mean about letting them down. It will feel hard now, especially to the loved ones closest to you, there is no way of avoiding that if you have a little bit of pride and respect for yourself. But the strength and trust of your inner circle is what you use to come back from this, they are the ebst people for you right now. I know they will be behind you on this and that is what matters most in this sort of situation. Use their support and just take the time to deal with this in your own head first and foremost. From th eimpression i got this is just an unfortunate situation that you found yourself in, nothing more than that. Once you start to deal with this yourself mentally it will all start to look better in time, but it is something that won't be easy to do.
And i suppose you know the situation you were in and the catalysts for being in that situation whatever all of those were. Perhaps the other thing which may help over time is not putting yuorself in that sort of position again, i use this as an example, not a opinion mate. If it was on a night out then just don't go there or to town for a good while. If it was something else, avoid doing that to avoid the situation, no matter how rare and random this was.
I don't mean anything to sound patronizing or anything like that mate, i really don't. I'm merely just typing my thoughts and hoping something helps in any way possible. hopefully something i said makes sense to you and just remember, it will look a lot better in the future, before the light comes the dark.