I could play Chess before I went into Kindergarten, (we call it Infant School here in the UK) and I Taught my Daughter how to play
blackjack before she started in Infant School. There is nothing wrong in teaching your kids Self defence before they are 18, many start Boxing, Karate and other forms of Martial arts before they are 18, and teaching them how to handle a deck of cards before they get suckered into playing such games in later life is just preparing them to defend themselves at a Card Table.
I remember my Daughter turning 18 and going to her first Party where there was alcohol being served, and although it is Legal here in the UK, it doesn't stop you worrying about them. We let her go on the understanding she Phoned home when she wanted to leave and I would pick her up.
When she did my Wife was talking to her while I got my shoes and coat and my wife started to laugh, I asked her what was so funny and she said, "Sara said Tell dad when he taught me to play cards, he did a good job, because I just rinsed all the lads at the Party"
When she got into the car I said "Thankyou, but remember this, I may have taught you everything you know about playing cards, but I haven't taught you everything I know, there's a big difference"