$1.$ Create a Python class named 'InvalidAgeError' that inherits from the built$-$in 'Exception' class.

$$a$.$ Define an $\text{'}\_\_$init$\_\_\text{'}$ method within 'InvalidAgeError' to customize the error message.

$2.$ Implement a function named 'validate$\_$age'.

$$a$.$ Define the function to take an 'age' parameter.

$$b$.$ Use a 'try' block to check if the 'age' is less than $18.$

$$i$.$ If 'age' is less than $18,$ raise an instance of 'InvalidAgeError'.

$$ii$.$ If 'age' is not less than $18,$ print 'Age is valid.$\text{'}.$

$$c$.$ Use an 'except' block to catch 'InvalidAgeError' and print the custom error message.

$3.$ Outside the function:

$$a$.$ Call 'validate$\_$age' with the following age values.

$$i$.$ validate$\_$age$(22)$

$$ii$.$ validate$\_$age$(16)$