I never do things like chest/shoulders/tri or back and biceps. I work from the larger muscle groups the smaller except for squats I like to finish my training week with those. The reason I dont do things like back and biceps is because your biceps get enough secondary work while doing back. Why further annhilate them that day and take the chance of burning them out too much?
If your telling me you can workout your entire body with the same intensity that I do my one bodypart a day then id call bullshit because ive been doing this awhile and sometimes after a chest workout im fuckin drained.
This is what the problem is. I never said you had to go in and completely destroy your target muscles. Systematic increases in poundages each and every lifting session will get you results. I'm not saying your way won't work, but I think it's not optimal for a 15 year old to be doing one body part a day splits. . Functional strength should be the basis anybody starting to lift should be targeting.
Once again, it's on you. But the science, strength coaches around the world, 50 guys on this site, about 6 thousand athletes all agree with me. And, if you don't believe them Lee Priest has a small article in this months ironman that states exactly what I said above.
But again, if one body part per day is what gets you in the gym and is something you enjoy then go for it.