What are your intentions with the meat? Any chicken can be eaten at any age, it’s how you cook them at different ages that is important.
Leghorns are never going to be a good meat chicken. They are small and just don’t have much meat on them. They were developed to lay eggs, not be eaten. They are real good at converting feed to eggs; they don’t waste much nutrition maintaining a big body. But people eat quail. Leghorns are bigger than that.
It’s like any other chicken. If you want to fry or grill it, get them pretty young. Different people have different tolerances but maybe 12 to 15 weeks. The older they get the slower they need to be cooked and the more moisture you need to keep with them.
It’s a personal preference thing. There is no “best” age that covers every human being on the face of the planet. How do you plan on cooking it?