The term for the act of sucking your own penis is Auto-fellatio and basically you either can do it or you can't.
If you start trying while you are still young then practice might make perfect. A supple body and a long penis would definitely help.
Some people find lying on their backs and raising their legs up and over their heads makes it a bit easier.
Don't force it, you could injure yourself that way, and that's the last position you want to be found in!
Most young boys can flex enough with practice to at least lick their penises, but taking it to orgasm while stretching your spine that much, is not so easy.
