There is certain age limit for each and every thing. Yes there are some exceptions, but the ideal age is already decided by filtering all the possibilities. If you go to any trading platform then definitely there is an age limit to trade. The reason for this is that, at 18 years of age, a child understands the responsibilities and sometimes start earning himself also. Trading should be done highly with own money only, so therefore all the trading platform allows only players above 18. You can teach your child about trading, if he is good enough to understand economics. Now when to trade or not and with whose money, completely depends on the parents.
A child can think about earning money before the age of 18, I think it's good when a teenager has a desire to earn money. But at this age, it should be a simple and understandable job for him, without the risk of losing all the money, since in trading this is likely to happen at first.
As for trading, I agree here, it should be done only after reaching the age of majority and with a mandatory understanding of the essence of this activity. To have an understanding of all risks and all responsibility. I would like to protect my children from such great stress, especially at a young age, and would try to explain how difficult this is.