At what age teenager is ready for custom footbeds (not racing)?
I mean footbeds which would last longer than one ski season.
Is almost 17 too early?
Any skier will benefit from an aftermarket footbed. These come at several price points and quality levels. If the teens foot is still changing, I can't see the value in purchasing a several hundred dollar full-blown foot bed for a recreational skier. I would recommend a Sole footbed which can be shaped and costs about $50. They are worlds better than the crappy stock footbeds that come with most boots. I don't use them in my ski boots as I am into the top of the line footbeds for my skiing. I do have a pair of Soles in all of my street shoes and they really make a difference. Most footbeds can be used in street shoes as well as ski boots to get extra utility out of them.
By 17 most kids' feet have stopped growing. Start measuring your teen's feet now, and measure monthly to see if his are stable.
If you get the same measurement a few times in succession, you should expect the footbed to last a while.
If they are still growing, but at a very low rate, you still might be able to get a couple of seasons out of an investment in footbeds.