I don't think that taking that kind of substance that young is good for you, unless you have clinically-determined sub-normal testosterone (T) levels. That being said, there are, to my mind, safer and less safe methods of "boosting" your T levels.
One is to simply dump more T into your system - simply put, take steroids. Aside of the risk of damage to your liver, kidneys, testes, just to name a few, the biggest argument against steroids is that the strength and size gains they produce are what I refer to as a "drug-induced" vs a "drug-enhanced" effect.
By that I mean that if you would get that big/strong eventually, the steroids just got you to that point faster/sooner and once you're at that point you can stop the steroids, keep working out and maintain your gains, that would be a drug-ENHANCED effect.
But this is not the case. When you quit the steroids, you will lose your size and strength gains AND THEN SOME, i.e. you will shrink and weaken to a point that is much less than you were before you started your steroid regimen. In this case, the steroids caused your gains, this is a drug-INDUCED effect.
There is another class of supplements that will boost your T and these are collectively known as prohormones or T precursors, i.e. the chemical building blocks your body needs/uses to manufacture T (e.g. DHEA, 1-AD, M1T). These, to my mind, are less dangerous and avoid the hazards of steroids since your body will just take as much as it needs and when it needs to manufacture more T. But I do not think even this safer supplement should be taken by a 14-yr old.