Verb question: should be imperfect or conditional?

Verb question: should be imperfect or conditional?
 

Q: Ellos ___ 20 años cuando se casaron.
A: 1) tendrán 2) tienen 3) han tenido 4) tendrían
Why the correct answer is tendrían? Shouldn't it be tenían? Why use a conditional here if it's a fact and not possibility?

The speaker is guessing how old they were when they were married. The only possible answer is number 4.

Both imperfect and conditional would work in your sentence, but imperfect isn't offerred as one of your choices. The resulting sentences have different meanings, as Rusty indicates.

Ellos tenían 20 años cuando se casaron. = They were 20 years old when they got married. (The speaker knows for a fact how old they were at that time; simple statement of fact.)

Ellos tendrían 20 años cuando se casaron. = They must have been 20 years old when they got married/They probably were 20 years old.../I think they were 20 years old. (The speaker does not know for a fact how old they were at the time, and is making a guess.)

I wasn't thinking straight, I was assuming that they knew the age, I forgot that they could be guessing.

Muchas gracias. I forgot that they could be guessing.