is it safe for teenagers to use?
what age would be ideal for starting creatine use?
Creatine is fine for teenagers to use - they do not need to use it if they are not anaerobic or resistance training, though.
An ideal age to start using creatine is ~16 years old - since that is the age when most gyms will allow an individual to use the weight equipment. I wouldn't recommend any younger to start on any supplements but a multivitamin/antioxidant (EVERYone should be using one of these, regardless of age and training status).
You have to keep in mind that creatine is in almost all of the meats you eat
Table 1: Food Sources of Creatine 
Food Creatine content (g/kg)
so particularly so long as you keep amounts added reasonable, and stick to , its not a bad thing
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Along with the creatine, might I suggest a side of "education"? Reason being, it seems as if a lot of folks show up after taking creatine and are complaining about various side effects that are in no way associated with creatine. It has great potential, but not THAT great of a potential to aid you or harm you. It's one of the most basic staples out there, and is pretty much safe all around. I just don't buy it if someone starts out by telling me "I was using creatine and my BP went skyhigh and my liver really hurt"....that usually tells me that there's either something fishy going on, or someone's mom read an article in Redbook.
Best of luck!
-RecoverBro Zombie Specialist and Paracord Wrangler
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creatine of course. look at the write up from LG Sciences according to them if a teenage male takes creatine his penis will become larger due to the higher DHT levels.