The entire thought process behind Duolingo is to teach people to be translators and it's therefore in everyone's best interest to keep the translation as close to the original meaning as possible.
Yes, "tener" typically means "to have", but in idiomatic expressions such as with age it's translated differently. Saying "I have 20 years" (tengo 20 años), sounds incorrect in English, so we change it to make sense in the translation. While saying "a minute has 60 seconds" isn't wrong, and so there's no need to change it.
The algorithm duolingo is using to tell us that is probably only a work in progress. And probably only parrots back what you type and what it expects. And at a later date real logic will be added. I've never tried to use numbers on duolingo so I do not know if duolingo accepts them or not.