I agree that the earlier you start them the better. I started taking my kitten out into the yard when she was around 14 weeks old. That way she got used to being outside with all of the different sounds, smells and textures, while still being in a secure environment. When she didn't bat an eyelid at the traffic or the dogs barking down the street anymore I started taking her for walks. She does still mostly ride in my son's stroller during walks, but when she feels confident and curious enough she jumps out and walks on her own
She did have some trouble getting used to the harness, but she is much better with it now, she really only notices it when she decides to have a good roll in the sun
I just bought her a figure 8 harness for now, when she gets a bit older I will be investing in a walking jacket for her. I didn't want her to outgrow a walking jacket right away, even though I know they make kitten specific ones. Seeing as she is only outside when I am there to supervise, a figure 8 harness is secure enough for now.