Let Avery's age be \( a \).
Then Sheridan's age, who is three times as old as Avery, is: \[ s = 3a \]
Grandma, who is four times as old as Sheridan, is: \[ g = 4s = 4(3a) = 12a \]
According to the problem, the sum of their ages is 96: \[ g + s + a = 96 \]
Substituting the expressions for \( g \) and \( s \): \[ 12a + 3a + a = 96 \] \[ 16a = 96 \]
Now, solving for \( a \): \[ a = \frac{96}{16} = 6 \]
So, Avery is \( \boxed{6} \) years old.
To verify, we can calculate the ages of Sheridan and Grandma:
- Sheridan's age: \[ s = 3a = 3 \times 6 = 18 \]
- Grandma's age: \[ g = 4s = 4 \times 18 = 72 \]
Calculating the sum of their ages: \[ g + s + a = 72 + 18 + 6 = 96 \]
This confirms our solution that Avery is indeed \( \boxed{6} \) years old.