Usually juveniles tends to freak out more easily when any unfamiliar event occurs. Maybe that is how their nature instinct protect them from predator.
From my personal exprience, i didnt handfed my blackthroated canary (BTC) all done by their parents. I started to tame them when they just weaned. I had them flying to me and perched on my finger, etcs when i offer them goodies. Usually i do this in the evening, then one weekend when i am free in the morning, i let them out to fly around and found myself laughing at one of them. Guess what... he got freaked out by his very own shadow
due to the morning sunlight... Mine too will freaked out easily when they hear any high sharp or hisss sound. Until around 3 months then they are more stable.Time, patience and passion are the main factors here. IF you don't have these 3 factors then it better not to do it. Another factor which we cannot control is their natural instinct. Their very instinct to socialize with their own kind to breed and to follow each other behaviours. Easier way is to have one bird that is very calm to act as a role model.
All the best to whatever clauses you are taking.