Ok...that's a good thing to remember, as I have a 4 week old baby that seems quite interested in pecking and chewing at stuff already, she was chewing my phone case just a moment ago, just wondered weather it's possible, but I'll be playing it safe just in case.
I don't want to force them so I will still feed them the formula but let them investigate other foods only when they are old enough as I don't want them to wean themselves if they still need some of the goodness out of the formula.
Their immune systems are not fully developed until they are at least 3 months old. weaning a bird early is just short changing them on fully developing immunity, which can make them more prone to secondary problems relating to stress.
since your bird has an interest in nibbling on things keep some millet and seed scattered on the cage/container floor and right before you handfeed it place your finger at the base of the crop. if you feel food in there feed less formula than what you would normally feed per feeding.
Giving them the millet and seed on the ground will just guide them in the right direction. I had 4 chicks that we raised this last year. From 3 different clutches. The first weaned at 6 weeks. The second at 7 and a half weeks. The last two were 7 and 8 weeks. They were all nibbling and exploring at 4 weeks though. They wean when they are ready if you let them make that choice. You are just giving them the opportunity to learn what they need to learn. They will learn it when they are ready. From everything I have learned since then.. I will still continue to give them the opportunity when they get curious.. but by no means get concerned when not weaned at 6, 7 or even 8 weeks.
You can teach them to eat formula off a spoon. My babies reach a point where they don't want to be restrained for feeding and don't want the syringe and that's my signal to break out the spoon. At first I have to shove it in their beak to get them to taste it, but once they figure out that it's yummy formula they're quite happy to eat from the spoon.