Learner-focused corrections for einundneunzig — typical errors vs. correct German, with short explanations.

• neunundneunzig
  einundneunzig
  Confusing the ones digit 'ein' (one) with 'neun' (nine).
• einundneunzig Jahre
  einundneunzig Jahre
  Incorrectly capitalizing 'Jahre' when it follows a numeral in this context; it should be lowercase unless starting a sentence.
• 91 Jahre alt
  einundneunzig Jahre alt
  Using digits in formal writing where words are preferred, though digits are acceptable in informal contexts.