Okay, I found a way to solve it... must have been very tired when I first tried, actually it is quite simple.
I got x = 1/2 * [y + sqrt(5y²+116)]. Then I told my calculator to find solutions for which x is an integer, and it got 1, 4, 7, 13 and 20 (Didn't want y to be greater than 25...)
1 seems to be the right answer, somehow I had missed it in my first try...
so the presumably correct solution is (6,1).
x3: (18,3)
prime numbers: (59,5)
divided: 11.8 (11 years, birthday: july 12th)
so you would be more or less close to 12 years and able to create this riddle... Not bad for someone that age, I'd say. But it means that the answer can be false too
So I'll try the third one: (13,7)
-> (39,21)
-> oh... starts becoming uncomfortable... oh, wikipedia has a list
(167,73)
-> ~2.288
-> 22.88 years (22 years, birthday: june 13th). I think this one is by far more believable, regarding what you have to know and use for creating this task and also regarding your behaviour, which does not fit a 12-years-old
(Sorry @everyone who is 12 years old. Don't feel attacked now!)
So I think you were 22.88 years old (yesterday...)
I liked it 
And it's 15, of course.
