Learn Icelandic: Sonurinn minn og dóttirin hans eru jafngömul. - My son and his daughter are of the same age.

The prefix jafn- attaches directly to the adjective gamall (old) to mean "equally old" or "of the same age". It is a highly productive prefix, meaning you can attach it to almost any descriptive adjective, such as jafngóður (equally good) or jafndýr (equally expensive).

Why is jafngömul in the neuter form if it describes a son (masculine) and a daughter (feminine)?

Whenever a plural subject includes a mix of different grammatical genders, Icelandic always uses the neuter plural form for adjectives and pronouns. Since the subjects are masculine (sonurinn) and feminine (dóttirin), the compound adjective must take the neuter plural form gömul, resulting in jafngömul.

Could I write eins gömul instead of using the compound word jafngömul?

Yes, you can say eins gömul or jafn gömul as two separate words to mean "equally old". However, fusing them into a single equative adjective with the jafn- prefix is extremely common, sounds very natural, and is the standard way to express this in Icelandic.

Why is the definite article attached to the nouns in sonurinn minn and dóttirin hans?

In everyday Icelandic, the standard way to show possession is to put the noun in its definite form (adding -inn or -in) followed by the possessive pronoun. While you can say minn sonur or hans dóttir, putting the possessive first without the article sounds highly formal, emphatic, or poetic.