As early as your kid shows an interest, get them started. A few lessons on the basics, the count, will pay dividends. To catch fish you have to learn to cast a little bit. Learning to throw is good, learning to fish is better. 20 years in law enforcement and 20 years guiding taught me that kids who learn to fly fish pull their heads out of their ass sooner and with a better outcome.
I always thought 10 years old was about the earliest someone could pull off a successful trip up in Bristol Bay, there were a few exceptions and those kids were fun. A 12 year old who really wanted to be there would be down on the dock after the day was done still catching fish. I was catching Grayling and Char when I was three or four and my first King when I was 6, but we lived in Alaska before it was a state so it was pretty easy for my dad to get me started.
There is good water all over this country, good guides as well. You don’t have to learn on Rainbows and Brookies. Investing a little money now may pay big dividends in the future. Speaking of the future there just ain’t nothing better than taking the grands out for a little fishing either. I think is great you wanting to learn as well.
Good luck.