The best time to "dispatch" egg type sex link cockerels is day old.
At no point do they have more value as a meat bird than what went into them.
As meat chickens free they certainly are not.
Truth is that we mainly exploit sex link genetics because it allows to discard the unwanted portion(Males) of a hatch at the time of hatch. As mentioned above almost all sex-link males are dispatched the day they hatch.
This is because the increase in laying ability due to hybrid vigor is the more profitable/easier venture. Or simply the sex-link pullet makes a commercially viable option as a layer hen while the cockerels make a poor option at best for the modern mass production model of commercial broilers. Under the overcrowded conditions of growing sheds that house billions of CornishX broilers in the US the sex-link males would likely kill each other before reaching market weight.
As stated by SD Lady Bird it is possible to get a decent meat animal from sex-link males. If your production model is such that the cost of production is lessened by foraging feed enough a profit can be made on sex-link cockerels.
There are several issues with the sex-link cockerel as a meat bird but the one I find to be the greatest is variability. There are just too many "Sex linked" crosses to generalize. Practically put any gold/red rooster over any silver hen and you got sex-links.
What does BillysFlock have? A production RIR roo over a white Leghorn hen? Likely, because most Sex-links are bred as layers not meaties. Which is why ChickenJerk is right to add "egg type" in his post.
My sex-links aren't bred to be "egg types" rather they are bred as "dual purpose". A WLR Cornish roo over: White Rock, Delawares, RIW, Silver Kraienkoppe, and SLW. They dont lay like a battery hen or grow like a cornish cross but both the males and females make a superb free ranging animal. Both egg and meat utility is what my production model calls for.
What type of chicken do your production goals call for?
Checkout this awesome thread for alot of useful info:
https://www.backyardchickens.com/t/261208/sex-linked-information