I personally will never let a cat outside for the first time on a harness. I had a very bad experience with BooBoo.

She was a pedigree so I wasn't going to het her out to roam. She would just be allowed in the back garden while we were out there to supervise her.

One day I decided to let her investigate and put her harness on. She was a little weary of both the outside and her harness (even though she had gotten used to it indoors). The door decided to slam shut a few minutes later due to a draft... well that of course scared the crap out of BooBoo and she flipped, she went absolutely mental and went wild. I of course thought I had better get her indoors quickly incase she jumped up the fence to escape. I brought her back in but couldn't pick her up she was all claws and did alot of damage to my legs face and neck. When she got indoors she just went on a frenzy around the house! ripping anyone that went near her to get the harness off.

You have to be careful with cats, they are unpredicable when frightened, they lash out.

It should be ok for your young one, as she is still young. BooBoo was just under a year when we did that.

I also find that cats are ok with harnesses until you attatch the lead

Sorry I couldn't offer you any advice on when to let your baby out. But just be careful with harnesses if you decide to use one. Get them used to wearing it indoors, then get them USED to having the lead attatched.