Yes, I have 4 boys. Ages 9,7,5, and soon to be 4years old. When I called up Sensei he said that he would teach 5years old and up. This surprised me. Thier very first class had them all hooked. By the 3rd class, I was hooked too. I asked if I could start in the next class myself. Now we are a very small dojo, and sometimes, though not often, we are the only 4 students in a class. Because I am a single parent, my soon to be 4 year old comes along too. He usually sits patiently and watches during class. I have even witnessed him doing some of the motions at home when he thought no one was watching. No of us had a gi and I didn't want to sink money into 4 gies(sp) if it was only going to be a short term fad, so I came up with the idea of the boys paying for 1/2 the price. They were always asking for extra jobs around the house and farm, so that they could buy thier gi. They were all able to buy thier gi within two months. I got one too, and we have been in classes for a little over 4 months. Not only are they in an activity that they love, but I got a lot of help that continued even after they were able to raise half the cost.
The sesei is wonderful with each of them, and though I know they are not comprehending the essence of aikido beyond the physical, it has all been introduced. We always have a partner to practice with at home, and most of our dinner conversation is on aikido. (It beets being Pokemoned to death.) And instead of settling for being mom on the side lines, I am involved too. I have found it fasinating, though I would never have tried it without seeing a few classes first. I never had an itch for any martial arts.
Sorry so long, but I really wanted you to know, at least in our case with a fantastic sesei, that children, even young boys, can start to grasp the fundamentals of Aikido.