Answer:
Jason is 14 year old.
Explanation:
Given: Joe, John and Jason are each a year apart in age.
Sum of their age is 39.
Lets assume the age of Joe be x
∴ Age of Jahn will be

And age of Jason will be

Now, putting up an equation for the sum of their age.
∴

Opening the parenthesis.
⇒

⇒

Subtracting both side by 3
⇒

Dividing both side by 3
⇒

∴

Hence, Joe is 12 year old.
Next subtituting the value of x to find age of Jason and John.
Jason=

∴Jason= 14 years.
John age=

∴ John Age= 13 years.
Hence, Jason is 14 year of age.