I've done that ride many times. Some sections have moderate traffic, but nothing to worry about. I did the ride the first time at 13 or 14, so a 16 year can do it. There is a section that you need to ride on the freeway or through camp Pendleton. Can a 16 year old ride the base alone, call and check. Do not ride the freeway. 90 miles is a serious distance I would advise you to find someone to ride with, just makes it more fun.