So I've had my beardie "Odin" for nearly a year now. He's just turned a year old and he's nearing 16 1/2 inches long. I've been increasing his greens diet with collard greens, green beans, basil and I switch up with bell pepper, carrot, and kale occasionally (Bell pepper is his favorite so I get that more often). He eats about 30 or so medium crickets a day. I do feed him a wax worm after finishing his greens every other day as well. He's just got out of his first brumination (and my first heart attack observing his strange behavior) and is shedding.
Now I know that when beardies are older they have a 80% veggie- 20% insect diet, but when exactly does that start? Is there an age range or size range? Right now I put his diet in a more 40% veggie- 60% insect since he doesn't finish all of the greens I lay out for him each day, but he's happy to eat crickets and waxworms all the time.
He will most likely start to make the switch on his own. At 16.5 inches he may still have a bit of growing to do so bugs are still important. However, you can always try offering veggies first and then waiting a few hours to feed bugs (if your schedule allows for that). Sometimes this "tricks" them into eating more greens.
Yeah it's a gradual transition and varies between dragons. It's usually between about 12-18 months but it sounds like he may have another growth spurt to go. I agree with Skipper's advice.