I wouldn't go as low as a 100, it wouldn't take long for you to outgrow the size of the bike and also be wanting more power. The lowest I would go is a 125, but thats when you start getting into supercross and motocross riding styles, as most 125 aren't geared for trails but more for tracks. If you are wanting to strictly ride trails, I would say get a fourwheeler, but not a utility. Utilities are fun and all, but riding a manual fourwheeler is funner, imo. Honda makes a 300EX thats really great for people that are first starting out, they don't have unbelievable power like the 450s and 700s and you can mod them out to give you extra power and speed as you desire until you are ready to step up to the bigger machines.