I think it's taking a very literal translation. "but SHE is too young for him," would be "mas ELA é muito jovem para ele." It means the exact same thing either way though, so I think both should be accepted.
"Muito" can be used to mean "too" in this case, because the context makes it clear that the woman is too young for him, not simply that she's very young (note the use of "mas"). Demais and demasiado are also correct.