Charak Samhita
वयोन्तमध्यप्रथमे वातपित्तकफामयाः । बलवन्तो भवन्त्येव स्वभावाद्वयसो नृणाम् ॥३११॥
In prathama vaya(childhood), there is predominance of kapha; in madhya vaya (youth and adulthood), there is predominance of pitta and in anta vaya (old age), there is predominance of vata dosha. These dosha have a natural predominance as per the age of the person.
english translation
vayontamadhyaprathame vAtapittakaphAmayAH । balavanto bhavantyeva svabhAvAdvayaso nRNAm ॥311॥
hk transliteration by Sanscript