I would say until they are in their teens and able to think for themselves in a mature manner.... in other words, not just being "lazy, don't wanna get out of bed" type of excuses.
If and when they are capable of having healthy discussions about religion, spirituality, etc.
No, I do not believe young ppl should be forced to go against their will... by ANY means ! This only produces resentment.
I am VERY grateful for my Dad... that it was "understood" that my sis and I went to Church every Sunday, (although my mother never did).. no questions asked, while we were growing up.
It gave me a foundation from which to question and explore further.
I was raised Catholic, yet once I got out on my own, I continued my quest and dabbled in quite a bit for about ten years until I came back full circle to my Christian beliefs... no longer Catholic, however.
I look at it this way.... it is GOOD to ponder, to question, to seek the truth.
Truth will stand on its own. Truth has nothing to hide, nothing to fear. It is a healthy thing to deisre truth and spirituality... and it can stand up to the questioning. If the heart is sincere in its quest for truth, it will find truth.
Seek, and ye shall find.