When I was young  I could curl my body weight.  I weighed 125 when I was lifting at the age of 18.  My strength to weight ratio was very high.  I can't even come close to that anymore.  As far as being safe yes it's safe however it's usually impractical.  I was very "strong" in my upper body, however I was not well rounded.  No lowerbody development and very poor stretching ability.  If you focus on one thing you can always increase it to crazy numbers but I would shoot for an all around fitness level.