I recently brought a new store-bought budgie into our flock. They weren't sure how old he was, but I would venture around 4 months. I watch him in the store over a couple weeks prior to picking him up and noticed he seemed to have a very sweet disposition. I think that helps a lot!
I can say from my experience it takes a LOT of time and patience, and a LOT of millet. I worked slowly at first, just holding my hand in the cage, gradually resting it closer while he seemed calm, and backing off immediately if he got spooked. He was very skittish over the first couple of months, but I spent about 10 min a day with my hand at first, then sitting and talking or hanging out near the cage whenever I could. Gradually, over another couple of weeks, I would use millet to coax him out to the door, and spent time just letting him nibble and associate treats with our interaction. It took a LOONG time for him to trust enough to step up, and I'm only JUST now, nearly 4 months in, trying to introduce training exercises. He's still a little scared of the clicker, but he now happily comes to the door and chirps for treats and attention when I open it up for play time (he often beats my Linnie, who was hand fed and is deeply bonded to me!).
The trick is taking baby steps every day. Watch their body language and be sensitive to their moods. A few minutes of calm, positive interaction with lots of love, patience and rewards every day will do MUCH more for taming than an hour of forced and stressful attempts at training! It takes a long time to earn a store-bought budgie's trust--even if it's a baby--but the reward is well worth the effort =]
good luck! please be sure to post pics and progress!