We use Duocal — the US cans do say that the babies have to be 12months or older. In Europe where the Duocal is made, there is no such warning.
We started using it when Matthew was 5months old and got the tube, to increase his calories. Kendramom (Leo) actually posted the same question on a different board and she got tons of responses saying that doctors recommended it for patients under 1 year in the US — so it is widely used for under 1.
I do not know much about the junior formulas because Matthew is still on infant formula. Really, by discussing it with every nutritionist at various feeding clinics, it really does not matter until they are a bit older, maybe 18months or so. The big difference infant vs junior formulas is the concentration. most infant formulas are 20cal/ounce if mixed as directed; most junior are 30cal per ounce (although there is even higher but only recommended for different diagnosis).
The Duocal you can mix in with formula but you can also mix in with whatever else he is taking orally — to me, it has no taste when you do not put too much; others say it is tasteless, others say they can taste it so go figure
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