On this, the 19th year of their birth, on the event of the coming of their age, Ruffnut and Tuffnut Thorston must take part in the most perilous trials of all: The Thorston Induction Trials!
— Gruffnut in "A Gruff Separation"
Gruffnut Thorston arrives on Dragon's Edge to referee the Induction Trials for the Twins on their nineteeth birthday. He claims that only one of them can pass. Initially both agree not to partake in the Trials, but Gruffnut convinces each one separately to take the Trials for the benefit of the other. The Twins' first trial is gathering Speed Stinger venom, the second gathering molted Changewing skin, and the third getting a treasure box from the lair of a Titan Wing Zippleback. Both Twins bring the box back but both decline becoming a full-fledged Thorston without the other. According to Gruffnut, they passed the 'secret' fourth trial, that of choosing family above status. Gruffnut takes the chest and the Twins return to the Edge. Once there, they meet Agnut Thorston who had flown in with Gobber to administer the Induction Trials. She indicated that Gruffnut had never passed the Trials himself and was not considered a full member of House Thorston. He had duped the Twins to get at the hidden treasure in the chest, which turned out to be a Dragon Eye lens.
Ruffnut: "Most Thorstons don't have that long of a life span, so if you actually make it to 19, it's a pretty big deal."
Tuffnut: "Yeah, and because of this, every Thorston, on their 19th birthday, must complete three extremely dangerous challenges. Each is specifically designed by the high council of Thorstons to test the mental, physical, and height limits of the Thorston taking the trials. Once completed, you're officially inducted into the clan, with all the rights, benefits, and costumery that go along with it."
Fishlegs: "And every Thorston does this."
— "A Gruff Separation"
The trials appear to be designed and tailored for each individual Thorston.