Transformation targets with に

The に particle can be used to mark the target of a transformation.¹

Example: This year I become 18 years old

ことし十八歳になる

Info

In this example, ことし (this year), 十八 (18), and 歳 (age) are marked with the に particle, making this the transformation target, what the subject is turning into.

なる is a verb meaning to become, used in this case as the subject is transforming, or becoming the marked transformation target.

This sentence has an invisible が subject, and as there is no sufficient context to suggest otherwise, it defaults to わたし (I).

Footnotes

1. Japanese from Scratch (8) ↩