Good question so heres my experience with my children.Forgive me I don't fly fixed wing but I think similar in many ways to rotary.My children started seeing the R44 up close from an early age as I used to land in our garden a lot. Although they were excited about it when I landed and shut everything down as my wife brought them over to the machine I was not that surprised I suppose to find my older girl aged 2 at the time was very cautious about the machine and would not even sit in it. So we didn't force the issue.My little boy only 1 at that time was not that bothered as you might expect. A year later when my girl was 3 I had it home again and this time asked if I would bring her flying,my little boy now 2 was very interested but like his big sis the year before would not go too near let alone sit in it. So I popped her car seat in the back diagonally behind me ( I sometimes find some peoples first flight in a heli because of the bubble window to the floor in the front can feel a bit exposed ), fitted her headset ( which incedentally we practised with standard earmuffs at home as a game a lot to get them used to the idea ) After a chat over the intercom at which she was very excited with I started and lifted into a hover and she was very happy and then gently away for a truly memorable flight which she loved.A year later you guessed it my boy now 3 wanted to come up so ,this time with the two children in the back and my wife in the front we went for our first full family flight and it was wonderful to hear my little girl now 4 of course telling her little brother all about what to expect !.
So I would say bring him flying when you feel confident and wait until he is ready too.
R