It's been said a couple of times in the books that it is a crime to keep an apprentice to long, but it still does happen. It made me wonder if the opposite is true. That some parens finish the training and pass an apprentice through a gauntlet early. This also may not be legal. (Though it may still happen regardless)
Now there are statements to the effect that apprenticeship is legally required to be at least fifteen years. Though HoH:Tl mentions that adult Hedge Magicians inducted into the order are an exception. So it may not be a law without loopholes. Canonically both Criamon and Bjornaer often recognize apprentices as full magi well before their 15 years are up. But this is apparently only official within the respective houses as they still pay lip service to the 15 year rule. But House Tytalus does officially recognize a Tytalan apprentice as a full magi if he slays his parens. So there may be some path around the peripheral code they've already navigated.
Though I admit it should be rarer then stalling a gauntlet there reasons a Master might try to rush an Apprentices graduation legal or not. The Parens might not have been as ready for the commitment as he thought. The Apprentice might have flaws that make him unsuitable as a lab assistant (Like they're an idiot). The Apprentice might be a political liability, or the master needs one more loyal voter at the next tribunal. That Bonisaugus the master hates might be threatening to take the student. Heck a Parens might just think the Apprentice is ready to be a full Magi and doesn't feel the need to keep them around as their lab monkey.
This could lead to some interesting characters and plot hooks.
A sheltered Criamon child living in the Cults Domus Magna. She holds some vital piece of information but expects to treated as a full Magus.
A young magi released after only 10 years of training suing his former master for the leasons he missed out on.
After that character's successful case a Hedge wizard inducted into the order demands his fifteen years of training from the Magus who invited him.
The nine year old Tytalus who managed to incinerate his drunken Parens with the man's own wand.
House Tremere requiring Parens to power train their apprentices two seasons a year but gauntleting them after eight years. Swelling their numbers without using fast time regios.
One thing I thought of after reading the Tytalans tradition of inducting other Magi (whether they want to be or not) into their house if a Magus defeats a Tytalan in some spectacular way. What if an Apprentice of another Magi defeats or kills Tytalus mage in a worthy manner. And the house inducted the Apprentice as a full member as they would one of their own Apprentices.