Using Bitcoin and altcoins under 18 is totally fine, however trading even though is currently allowed, one might not be prepared mentally for the kind of pressure that comes with it. Technical Analysis can be learned at any age, but the biggest enemy when trading is psihology, yourself, and the difficulty of risk management. Master yourself first and then you can learn to master trading.
Well, I agree with your statement. Yes even below 18 still can handle cryptocurrency as long as he/she know how to count and what is the real meaning of value as a currency. There was a news that I saw in the article that Teenage bitcoin millionaire Erik Finman, 19, has advice for young people looking to invest in the cryptocurrency. He has invested bitcoin at the age of 11 years in 2011 with the help of his grandmother.
So, there's nothing wrong with the age as long as they know how to compute.