Up until what age?
Well it depends, doesn't it. On the child, on the circumstances, on the relationships, on a host of different factors - as 2rebecca and Countess illustrate. I think OP made it clear in her first post that there was a bit of an agenda here. MIL could have taken a balloon ride anytime if she was that desperate to do so. There is no reason she has to do this with a grandchild. Not taking no for an answer and nagging away for three years is manipulative, unpleasant and unnecessary - and bound to cause ill-feeling.
In arguing that "a 13 year old should be able to decide whether to go on a balloon ride himself" you completely (deliberately?) miss the point. Yes, on the face of it - but no, if it is against the parent's wishes.
Are you seriously suggesting that there must be a "cut-off point" at 13 or some other age (regardless of the maturity of the child) when you felt it was fine for your DC to go off and do what they wanted with others, without your approval? I would say that is dangerously naive and frankly inept parenting. It makes it much, much harder to draw the line if things start to go wrong.
I don't know whether you are projecting or speaking from experience when you say "Wow! Good luck when yours reach teenage years!". What I can say is, if you have let them set their own boundaries far too early your task will be much, much harder. You'll need more than luck - and so will the children.
I think your attempts to imply that OP is some kind of irrationally fearful control-freak are completely wide of the mark. Your further argument, that there is a precise age at which children become responsible for organising their own time regardless of their their parents' disapproval is frankly silly.