I don't know enough about this, and was hoping that there would be some more answers. I would think that steroid use could be dangerous and felt concerned when I saw the article by Eichner and Tygart below: Adulterated dietary supplements threaten the health and sporting career of up‐and‐coming young athletes.
Somewhere, I read that young people were not drug tested, which would - at least - draw attention to adulterated products. I looked to see what I could find online in the UK, that might give you some thoughts of others:
Herriman, M., Fletcher, L., Tchaconas, A., Adesman, A., & Milanaik, R. (2017). Dietary supplements and young teens: misinformation and access provided by retailers. Pediatrics, e20161257.
Parnell, J. A., Wiens, K., & Erdman, K. A. (2015). Evaluation of congruence among dietary supplement use and motivation for supplementation in young, Canadian athletes. Journal of the International Society of Sports Nutrition, 12(48).
Eichner, A., & Tygart, T. (2016). Adulterated dietary supplements threaten the health and sporting career of up‐and‐coming young athletes. Drug testing and analysis, 8(3-4), 304-306.