Write down the given ages and the mean.

The ages are $8, 11, 8, 10, 5, 7, 2u, 11, 11$.
The number of ages is $n=9$.
The mean age is 9 years.

The mean is the sum of all values divided by the number of values.

So the equation becomes:

Multiply both sides by 9:

Subtract 71 from both sides:

Divide by 2:

The value of $u$ is $\boxed{\text{5}}$.