Answer: 54


Symbols
Suppose the youngest dwarf is $x$ years old.

Sum of the ages of the youngest three is $x+(x+1)+(x+2)=3x+3$
Sum of the ages of the oldest three is $(x+4)+(x+5)+(x+6)=3x+15$.

We know that $3x+3=42$, so $3x+15=42+12=54$, so the ages of the oldest three of the Seven Dwarfs add up to $54$.


Patterns in the numbers