Bitcoin? I mean why would you want to make it confusing. If you are 15 years old, and let's assume you retire at 65, and the rest is irrelevant if you do not want to, then we are talking about 50 years of working and investing.
If you do just bitcoin and nothing else, sure there will be some other coins that go up and down, but you can ignore it when you have that much time. Your age is to your advantage, I am much older than you and do not have the same time as you do, and still do long term bitcoin holding. Just work hard in your regular life, earn a salary, and put even the tiniest about you could, for 50 years, by the time you are 65, you could literally live off that much money.
This, there have been some studies about this in the past and what was found is that an early and easy retirement had less to do with how much you earned and it had to do more with how much you saved, invested and how early you began, someone that can begin their journey at 15 years old and invest in bitcoin for decades will most likely do great and have an easy retirement, so there is no need to try invest in other altcoins when something like what is happening with luna could happen to you and you could lose decades worth of savings in just a few days.