This is unfortunately an old wive's tale which is popular amongst too many adults and children. You're not going to be able to lift more heavier weights until you've reached puberty anyway, when your hormones will kick you into growing taller and growing muscles, and your mindset also becomes slightly more mature and stronger. What I mean by this is that your mind must be strong for you to lift; at the same time, Weightlifting can make your mind stronger. Therefore you need to start with a weight that both your mind and your body can handle (i.e. weightlifting is partly psychological, partly down to muscles).
Whatever you can safely lift at 12 I do not believe will stunt your growth. Running and jumping create more forces on the body than weightlifting does. The US government observed that soccer, Basketball, football, Skateboarding, and bicycling injuries cause most stunt in growth. Weightlifting will actually stimulate your growth, including your bones and muscles. The other thing is that the best weightlifters are naturally shorter and heavier - you should not confuse cause with effect: shorter weightlifters can deadlift, squat and over-head press much more easily as they have less work to do.
There have been many people such as Shaquille O'Neal, David Robinson, Karl Malone, Michael Vick, Dave Draper and Arnold Schwarzenegger all started weightlifting in their early teens. At the age of 12, it is highly unlike you will be doing 90kg deadlifts or squats, let alone clean-and-press. Weightlifting is a very safe sport (check the statistics for sports-related injuries). At the same time, it is not really expected of children to lift weights - starting off with sprints and callisthenics/gymnastics is a good foundation for building strength.
Start off with callisthenics for a year - have a look at guys like Ross Enamait. He combines weights with callisthenics (bodyweight exercises); and he even wrote a few books on callisthenics-only workouts to build stamina, strength and power.
Then move on to light weights (e.g. 10kg-20kg) for another year. If you aim to lift your own bodyweight first, perhaps for the third year, for a year or so, then you're not likely to go wrong. Don't try to go too fast - serious weightlifters look at targets in terms of years rather than months or weeks. They stick with the same weight for 2-3 sessions, and add on 2kg-4kg at a time. Have a look at Pavel Tsatsouline's book "Beyond Bodybuilding" (it has a load of good information), eat well and also have a look at Brooks Kubik's Dinosaur Files (which are really motivational newsletters), and also his books "Dinosaur Training" and "Dinosaur Strength Training Notebook" which have a LOT of useful information on weightlifting and sample routines.
Always warm-up at the start - this gets your muscles loose so they won't snap under pressure. Try deadlifts, squats and bent-over rows - perhaps even good-mornings and barbell curls. Try clean and press - avoid the bench-press, because you can develop more injuries (including rotator-cuff injuries and also dropping the weight on your throat). With standing exercises you do not need any spotters (people to help you in case you fail). The exception may be when squatting, but you can generally drop the weights either in front or behind you.
These are generally safe compound exercises (except for the curls - which is an isolation exercise). Compound exercises are good for stimulating all-around body growth. Your workouts shouldn't last much longer than 45 minutes; perhaps 1 hour maximum including rest intervals between sets/time when you change plates for different exercises. Have rests of 3-5 minutes if lifting very heavy weights (ones that you can't do for more than 3-5 reps with good form). Also, try having at least a day's rest in between weightlifting days. You can also have callisthenics days in too. For instance:
Day 1: Weights
Day 2: Callisthenics + sprint at end
Day 3: rest
Day 4: rest
Day 5: Weights
Day 6: Callisthenics
Day 7: rest
Day 8: Weights
Day 9: Callisthenics + sprint at end
Day 10: rest
Day 11: rest
... and so on.
If you want more proof? Look at American Football school programs - are the jocks short? No. Not unless it's in their genes to be short. They tend to start serious weightlifting at 15. But like I said, people have done it much earlier and still grown to be taller than average. What about Olympic weightlifters? They start at a very young age - it's the only way to get ahead in any Olympic sport. They are not midgets.
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