Would Qi Gong be good for someone with very little energy to do ?

I'm a skeptic on the whole chi thing being a magical force. Just letting you know where I'm coming from. With that said, qigong would be a good practice if you are like "elderly" weak. It keeps you moving and stretching, and I see a benefit in that for the people with that level of strength.

If you are young and just not as strong as others, then I would just suggest you start doing some athletic activities. Like workouts probably bore you or you would be doing it already. So I'd suggest sport, which is an excellent and fun way to get into shape. Martial arts would be an option for this as well.

But for young person qigong is probably too low level of activity and your joints are too nimble to benefit from such training. If you like qigong and really want to do it, then I would rather suggest that you try looking into taijiquan(taichi), baguazhang, or xingyiquan. These arts may be better fit for you and they may hold that something that you liked about qigong.

There is qigong for health, and there's qigong for martial arts.

Qigong for health is as meditative as it is therapeutic. All your doing is focusing your attention into a single point, and breathing. It's mental training for developing focus and self awareness, as well as aiding in recoveries. I believe qigong should be practiced by everyone in some form or another.