Pertumbuhan bayi ini normal untuk seumurannya.

English Translation

The baby’s growth is normal for his age.

A native speaker explained the sentence structure here on HiNative. The complete sentence is

Pertumbuhan bayi ini normal untuk bayi-bayi seumurannya.

The preposition “untuk” (for) seems to be odd directly followed by an adjective/adverb “seumuran” (be at the same age group). “Bayi-bayi” is obviously the noun to bridge between the preposition and the adjective/adverb. In this case, native speakers often drop the noun.