Answer:
Jason is 14 year old.
Step-by-step 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.