Each child is different, but I would suggest anywhere from 3 upwards. Older children will be able to focus for longer, but there is benefit in starting young, while a child's brain is developing. The sweet spot is probably from 5-10, but older or younger are definitely able to partake and benefit.
Does this require any prior experience or knowledge
None at all
Is the course affiliated to any religion
How long will it take to complete
You could probably complete it over a day or two (in short bursts), but spreading it over time could help it to soak in. You can keep returning to the course as often as you like.