"She will be twenty years old in three months."
Translation:Ŝi estos dudek-jara post tri monatoj.
The only difference is that dudek-jaraĝa is more specific. Dudek-jara is clear in the context and so would mean the same thing.
The word hints are hints, not answers - but most relevant in this case is that (as I understand it) they're created on a word-by-word basis, and not connected with any particular sentence.
IMHO, the English expression "in three months" is illogical compared to post tri monatoj. In Esperanto en has to do with being inside something. We're not doing anything en those three months. We're waiting for those three months - and then AFTER (post kiam) they pass... and then AFTERWARDS (poste)... and then AFTER those months (post tiuj monatoj) - she will be 20.