I think it really depends on how much your child can concentrate and how much you want to treat it as a formal education. I have a brother who's 10 and an exceptionally bright little dude. He asks me to teach him programming but whenever I try to teach him he soon grows bored. He also plays cello and has mastered it much more than programming. I think this is partially due to my parents making him practice every day for a set amount of time. If you make Alex practice his programming every day then you could probobly start teaching him around age 6 or 7 and he'd get great at and enjoy it a lot. Of course if he doesn't enjoy it you might not want to try to force him to do it at all.
I remember when I was around 5 or 6 my dad would teach me really basic math operations which I just loved. Every few evenings I'd ask him to teach me some math and he'd show me how to do some addition or subtraction, and then later multiplication and division. I thought this was the coolest thing ever because I was learning math which made me feal extremely proud and happy.
Alex might even ask you about programming. I know that I was always very interested in what my dad did at work and for play.
-Ben Jacobs (dooberwah)
PGP Public Key: http://dooberwah.perlmonk.org/mykey
"one thing i can tell you is you got to be free"