It is not to early to desensitize her to the harness-just put it near and let her play with it. I caution you, however, that the instructions that come with it can traumatize your baby. I followed the ones that came with my brand X harness and poor Corsair

spent the whole "first trip outside" trying to chew her way out of the thing. After that, I got an Aviator, but when I would hang it on her training perch, she would walk over and drop it off the end. Cute, but sad. It took months to get her to let me put it on, and she still hasn't built up much duration.
Use Michael's instructions from the start and you will be better off
http://theparrotforum.com/viewtopic.php?f=15&t=1329Just remember that babies will let you do things once that they really don't like and go slow.