If being an airline pilot is what you want, a cadetship is the fastest way to get there.
If you want, and can afford to fly beforehand, then to the best of my knowledge that won’t put you at any kind of disadvantage to be selected as a cadet. Bear in mind that you most likely would have to repeat any lessons you’d already done as most cadet programs start at lesson 1, regardless of prior experience.
At 15 I also only wanted to be a pilot. Consider the future of the profession - downward pressure on terms and conditions, lots of missed birthdays, Christmases, family events. At your age automated airliners starts to be a real concern too, how will the next generation of airliners look? Single pilot? No pilots? Who knows, but I suspect it’s coming.
I feel lucky that I’ve had a relatively good career so far, but the good jobs are becoming fewer and farther between. Unless you have a green card. If you’re open to moving overseas, have a look at the thread regarding Aussies working in the states. If it’s still an option in a few years it’s worth considering.
I would actively discourage my son from going into aviation. I would encourage him to do something where there is more likelihood of him becoming his own boss, even if he didn’t want to run his own business at least he’d have the option.
All that said when I was 15 almost nothing could’ve discouraged me from choosing this career, but that’s my take on it after 20 odd years in the industry.