I know that you are right; for many people English is a "half-way" step from their native language to the one they are learning. To recognize the difference is good, but the aim is not to produce word-for-word translations but acceptable forms in the target language. The English comparative is "older."
There is the rule of three syllabuses: if there are less than three of them, we use -er, -est; otherwise, more, most. And there are five exceptions: good better best bad worse worst far farther farthest many/much more most little less least (don't confuse with few fewer fewest which is regular)
Definitely not acceptable! Since when was the comparative of a single syllable English adjective formed with "more"? Come on, Kiwi, "older" is the way to go. "Became" is also rather strange, as we would probably just say "grew older", but I also put "became" as I feared losing a heart if I strayed from the obvious literal translation, Duo being what it is
If an English adjective is a single syllable then the comparative adjective is former by adding "-er". It is only for some adjectives with two syllables and those with three or more syllables that the comparative is formed by putting "more" in front of it.
"Elder" is generally used to refer to someone whose birthday precedes that of someone else e.g. elder brother, he is my elder etc. When referring to the same person at different stages of life that comparison cannot apply. In that case we use the adjective "older", not "elder".