A child enters iḥrām before puberty, reaches puberty during the pilgrimage, and simply continues the original iḥrām. Under the quoted rule this does not fulfil the obligatory Hajj.
If there is a valid renewed iḥrām before ʿArafāt, that is a different case discussed by Hanafi commentators and should not be conflated with the first.
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Kitāb al-Ḥajj: Obligation and Conditions of Pilgrimage
A child enters iḥrām before puberty, reaches puberty during the pilgrimage, and simply continues the original iḥrām. Under the quoted rule this does not fulfil the obligatory Hajj.
If there is a valid renewed iḥrām before ʿArafāt, that is a different case discussed by Hanafi commentators and should not be conflated with the first.