1. He became a president when he was 48 years old.
2. He became the president when he was 48 years old.
3. He became president when he was 48 years old.
4. He became President when he was 48 years old.
5. He became the President when he was 48 years old.
6. He became a President when he was 48 years old.
-----------------
Will you look at the word 'president'?Which ones are grammatical?
"a president" -- very general, referring to a presidency of just about anything -- a country somewhere, a corporation, or ???
"the president" -- This refers to a specific position depending on what the context (the other sentences and paragraphs around it) is.
You'd use "President" only when referring to a particular office and the whole title of the office is given, with proper capitalization:
President of the United States
President of General Motors Board of Directors
...