I don't think there is any maximum age nor am I aware of any religion that has a maximum age for joining.
If parents have the view that one religion is no better or no worse than another, they will probably be hands off as you suggest.
That said, isn't raising one's children to believe that one religion is just the same as another itself an attempt to instill a particular religious point of view in the children? Doesn't it perhaps imply that the children should believe that their choice of religion is as inconsequential as whether they end up preferring McDonald's restaurants to Burger King?
I think that saying that parents should just "teach their children right from wrong" begs the question. Who is to determine right from wrong? To a JW parent, what's right is for their children is to become JWs and what's wrong is for their children to do anything else.
Parents who have a religious faith which they believe to be the truth (to the exclusion of other religions or religious systems) naturally want to teach that truth to their children in their formative years.
Depending on what the parents believe happens after death, they may also be concerned with what eternal destiny the child will have if they should die young (but after they are able to understand right and wrong). This would strong argue against a "leave it up to them when they are adults to figure it out" approach.
Even if parents don't believe one religion to be the truth or don't have a religion at all, it's likely that they would want to protect their children from getting sucked into a destructive cult. Given the circumstances, a total hands off approach might be counterproductive. Imagine, for example, parents who see their 16 year old child starting to attend a KH or go to Scientology meetings with friends. Should they override the children's choices or leave it up to them?
In terms of religious training, there is always a tension.
Parents naturally want their children to make choices the parents consider right or at least avoid decisions the parents consider destructive.
On the other hand, wise parents understand that mature people need to make their own choices, that trying to force religious convictions into their children is counterproductive, like trying to force someone to love someone else.
(I was going to say that it is impossible to force genuine religious convictions on someone, but I'm not sure that's completely true. As we know, people are often indoctrinated into cults through mind control techniques. Such people can be genuinely and fanatically devoted to such a religion even though they got there and are kept there by fear, guilt, and misinformation.)
