Yes, a 10 week old can eat it too - compare the back label of your starter/grower and All FLock, the numbers are quite similar.
"Best"? No idea. Depends on availability. Doesn't matter what is "best" if its also not readily available.
Personally, I use the Nutrena Naturewise AllFlock Pellet in the 40# bags. Runs about $16.50 a bag at
TSC, a bit less at TractorKing (not one nearby) and I just signed up with Nutrena in hopes of getting their $2/off coupons. I've had birds on it as young as 4 weeks. I like the pellet, less mess.
When I can't get that, I use the Purina FlockRaiser crumble - 50# bag, $18.
DuMor has an "All Flock" which I've never used. 50# like the Purina, priced like the Nutrena - but its a 17% Protein, not the 20% in the Nutrena and the Purina. Since I have meat birds, that's a deal breaker.
If I can't find any of those (as has happened this year), I've used Nutrena's "Feather Fixer", another 20% Protein bag.
If I JUST had layers, no meat birds, the DuMor would be adequate - I free range, so they can hunt bugs to their little heart's content, about 13-14 hours a day during the summer, less in winter.
/edit I keep (buy) a bag of Starter Crumble for new additions to the flock - have the 6 SLW on it now. Will transition them to the All Flock before I integrate them with the main body of birds. For now, they are all in, effectively, quarantine till they bulk up and start losing that baby down. Then I'll have them together apart, in their own day enclosure, surrounded by the bigger birds while introductions are made (2 week process).
