His father diedwhen he was two years old. The underlined clause is a/an-

An adverbial clause is a group of words that functions as an adverb in a sentence.
The clause can modify or describe verbs, adverbs and adjectives.
In the given sentence, the clause ‘when he was two years old’ describes the time of the action of the verb ‘died’ . So, it is an adverbial clause.

Source: yourdictionary.com