In my first attempt at this one, I used "budou" instead of "jsou," since the question was about a future state. Was that totally wrong, conceptually, or just wrong because it's not a literal translation?
I don't see a future state in this question. It is more about old individuals of this species in general. I can't think of any reasonable Czech equivalent with the future tense either.
Thanks to both of you for the clarification! A followup: If the English original had been "How fast will these animals be when they are old," would "budou" have been appropriate, or would "jsou" still be the right verb for the Czech sentence?