Yes........I think it depends on how you teach the art. You can not give a loaded gun to a 5 year old. Even a teenager needs to be instructed on how to use it. Many schools don't take kids. The Aikido school I trained at had a rule about kids, no one under 12, yet my son started at 5 or 6, so they really didn't follow the rules. However, kids were not separated or encouraged to continue, actually made harder. My Sensei came to my son and told him the test was going to be in Japanese. My son freaked out and wanted to leave. I told him he knew all the Japanese terms, the Sensei was 'messing with him' actually Sensei was hoping he would back out. So it happens. I started my dojo just for kids when I saw how kids were treated in the Aikido dojo. I know many that have kids classes but its a tumbling class not an Aikido class. You don't want to have a 5 year old with a group of adults all the time. The child does slow things down, but on the other hand, they should be offered training. So I think some parameter should be met, where kids are separated. From 12 on, I see no reason why a student can't train with adults, but then again 12 is an arbitrary number as some kids are more mature at 12 than others, so its hard to say when its ok . The best solution is to separate kids from adults until they mature to train with them. I know mnay adults would not work with my son, because they felt he slowed them up. However others felt they could learn great control from my son.SO it really depends on the person in the class and the person training. I think alot of times Black belts don't want to train with beginners no matter how old. For my money 14 is a fine age to get started.