There is no magical age where babies are ready to tolerate milk. My son wasn't ready until he was nearly 14 months old.
I used a milk-based formula (Similac) and when I started adding milk, it was only a half-ounce at a time. I also gave him other milk-based products before trying milk - Yobaby yogurt and cubes of cheddar cheese (he had all of his teeth by 9 months). Once I introduced milk, I added it at a very slow pace because my son's stomach is easily upset. I added the 1/2 ounce then waited 5-7 days to see how he was really handling it before I added another 1/2 ounce. After he was drinking 4oz of milk mixed with 2-4oz of formula, I pushed it to increasing by a full ounce.
Doctors reccomend waiting until a baby is at least 12 months before introducing cow's milk because their digestive systems are still developing and pushing it on them too early can sometimes lead to lactose intolerance - even in babies drinking a milk-based formula. I did not use Soy with my son since he has no problems with the milk so I can't offer an opinion on that. I'd suggest waiting until your son is over a year old, then trying again with a very small amount of milk and see how that goes.