Arnold will play on the wing for St Helens in the Silk Cut Challenge Cup final on Saturday. This is a young man's Wembley - and the aplomb with which he occupies centre-stage suggests that this is one young man who is ready for it.
Fittingly, for someone whose hobby outside rugby is tending his aquarium, he thoroughly enjoys being in the goldfish bowl. "I never expected any of this to come so soon," he enthused. "Six months ago, I'd just played a few first-team games - and now all this. A lot of it's down to luck."
But Arnold has made most of his own luck, recovering from the disappointment of being left out of Saints' semi-final side to make it virtually impossible for Shaun McRae, their coach, to leave him out in the foreseeable future.
Given his chance because of an injury to his great friend Joey Hayes, he has scored a phenomenal 10 tries in the first four rounds of Super League, leading the charts ahead of vastly more experienced players.
"I told him that I had left him out purely because of lack of experience and that what he had to do was prove me wrong," McRae said. "And that's exactly what he has done."
Arnold admits that even his healthy self-confidence was dented by that semi-final omission: "It hurt me badly at the time. But that's what it's all about in this game; every time you get a knock, you've got to come back."
The young player who has come back in such devastating style was nearly lost to St Helens in his mid-teens. "I was training at Wigan, but they were humming and hawing about offering me a contract," he recalled. "Once Saints invited me down here, my mind was made up. There was no doubt about it."
Although he lived closer to Warrington, Arnold had been brought up as a St Helens supporter by a father who had once lived across the road from the ground. On such complex local loyalties are made decisions that shape careers - and win cups.
Saints have been the right choice for Arnold on two scores. He has been fortunate to be part of an outstanding crop of young talent, including his friend and rival Hayes. "It's a credit to the youth development and the scouting system they have here," he said. "And it has helped me to settle into the side."
The St Helens style of rugby also suits him perfectly. "We throw the ball around a lot, which is perfect for a pacy person like me," he said. Arnold has shown how devastating that speed can be in an attacking context, particularly with an 80-yard try from a standing start at Leeds two weeks ago.
Equally important - and perhaps symbolic for Saturday's return contest - was the way he ran down Bradford's Robbie Paul to make a try-saving, and game- saving, tackle in the Super League contest between the two clubs last Sunday.
Arnold is not just a try scoring phenomenon, although such prolific feats at such an early age inevitably dominate assessments of him. Many of his tries this season have been the result of arriving in the right place at the right time to go the last five or ten yards and score. That is not something that can be taught: Arnold has the genuine instinct.
"No one has said to me that the shirt is mine permanently - not with players like Joey and Alan Hunte around. But that's fair. I view it as a challenge."
Arnold relishes that challenge and the attention it brings. When other Saints are starting to look a little weary of answering the same questions for the umpteenth time at the pre- Wembley press call, he is still prowling bright-eyed and eager, just in case there is anyone who hasn't interviewed him yet.
The photographers want shots of him and Hayes fighting for the No2 jersey and, just for a second, there is a flash of the serious purpose that underlies the friendly rivalry: "It's the other way around now, isn't it?" Arnold asserts. "He's got to get the shirt off me."Reuse content