My oldest boy Henry is almost 7 and still has some funkiness in the way he pronounces words. Usually all it takes is a simple reminder, and he stops saying “lellow” and remembers to say “yellow.”
The word “first” is a completely different story. He simply cannot pronounce it any other way than “forst.” (Is he German?) Reminding him doesn’t work, modelling it doesn’t work. Saying “It sounds just like ‘thirst'” doesn’t work as he also says “thorst.”
The only way I’ve found to get him close to the correct pronunciation is to tell him to say it like a pirate. What does that mean? Well, you just insert a “grrrrr” noise in the middle.
So now when he says “forst” I say “say it like a pirate!” and he says “frrrrrrrst!”
And that, my friends, is why I will never be a speech therapist. But who cares. I have a kid who talks like a pirate.