I was diagnosed at 17.something. Please don't worry though, getting back on track with diet, exercise and maybe a bit of medication will see him right. I have been diagnosed for about 3 months and I have regular BG readings of lows at 5 and highs at 11. I still have lots of work to do but these forums have been a godsend. Ask any questions you like, loads of people are here to help.
The first thing I did before I got my appointment with my diabetes nurse was to get a glucose meter and to check my levels before and after food, I cut out carbs and sugar and upped my exercise. By the time I had my appointment and was prescribed Metformin I had already managed to bring my blood sugar levels down considerably.
If I can do it anyone can.
Good luck and welcome. Everything will be ok.