Well, I know that at a certain age, maybe by 10 years old, kids become critical of each other and maybe his friends would make fun of him for having dinosaur claw toe nails, like it's babyish. Honestly, there's no "too old" if you don't mind what others have to say.
As for nail polish on boys, I'm old fashioned in that I never considered putting nail polish on my boys. That's for girls. The only boys I see wearing nail polish are the goth boys, and they wear black, and that's out too. But if others want to let their sons wear nail polish, more power to them!