Translation:She has been writing books since she was fourteen.
The perfect substitute for "roků" is "let" which is more frequent and thus more often used for 5+ years translations. For 2,3,4 years> 2,3,4 "roky" is used more than 2,3,4 "leta".
The hover hints suggest "their" instead of "she" when describing who has been writing the books, misleading learners.
Those hints are general, like a dictionary, and do not consider the current exercise.
I used "He is writing books since he was 14 years old" and got it wrong. How can I say the above sentence in Czech?. I understood píše as present and deduct the English tense by context. Thanks
The above sentence does not look like correct English to me. I think the present perfect tense is required.
As VladuFu said, "He is writing books since he was 14 years old" is not correct; "has been writing" is what's needed here.
“He’s been writing books since the age of fourteen.” was not accepted. At least in Am English “...since the age of 14” and “...since he was 14” are both grammatically correct, are both used frequently, and have the exact same meaning.
Note that Czech also has a form which is closer to "since the age of fourteen": "od věku/stáří čtrnácti let" or "od čtrnácti let věku/stáří".