For danta is substituted dat in a bahuvrīhi samāsa when a numeral or su precedes it and the whole word expresses a stage of life i.e. an age.,
The ऋ of दतृ is indicatory, making Rule 7.1.70 applicable, by which a nasal is added before त् in declining these words in sarvanamasthana cases: and rule 4.1.6 also applies to them. All rules relating to उगित् apply to these. Thus द्वौ दन्तावस्य = द्विदन् \a child of the age in which two teeth have appeared\. So also त्रिदन्, चतुर्दन् ॥ शोभनादन्ता अस्य समस्ता जाताः = सुदन् कुमारः \a boy of the age in which all his teeth have appeared beautifully\. Why do we say denoting 'age'? Observe द्विदन्तः कुंजरः ॥,