English Translation
The baby’s growth is normal for his age.
Note:
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.